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THE ELEMENTS 



OF 



DYNAMIC ELECTRICITY 
AND MAGNETISM. 



BY 

PHILIP ATKINSON, A.M., Ph.D., 

AUTHOR OF "elements OF STATIC ELECTRICITY" AND " THE ELEMENTS OF 
ELECTRIC LIGHTING." 



FO UR I 'H ED I TJON, 




NEW YORK: 

D. VAN NOSTRAND COMPANY, 

23 Murray and 27 Warren Sts. 
1903. 



LIBRARY of CONGRESS 

Two Cooies Received 

MAY 21 I90r 

Cepynitirt Entry 

CLASS XXC, No. 

COPY B. 



Copyright, 1903, by D. Van Noptkand Co. 



F^eceived from 
Copyright Office. 
f3Je'07 



INTRODUCTION. 



This book was written for learners rather than for 
the learned. Previous to the last decade the demand 
for electric books was confined chiefly to scientific in- 
vestigators versed in the higher mathematics, and the 
authors of such books were electricians of the same 
class, who recognized the importance of mathematical 
accuracy in treating electric phenomena. Hence mathe- 
matical formulae became a prominent feature of such 
books. But the various electric industries to which the 
recent unprecedented electric development has given 
rise, have given employment to a numerous class of 
persons to whom mathematical books are almost un- 
intelligible, and yet to whom a scientific knowledge of 
the various kinds of electric apparatus which they are 
required to operate, or with which their business is con- 
nected, is of the highest importance. There ic also a 
class of liberally educated persons who desire to extend 
their knowledge of electric principles, but have not the 
time or patience to follow the intricacies of mathe- 
matical formulae, especially in the abbreviated form 
usual in the books referred to. A third class are stu- 
dents who intend to become electrical engineers, to 
whom a thorougli knowledge of elementary, physical, 
electric principles is important as a preparation for a 

ill 



IV INTRODUCTION. 

more extended mathematical course. To meet the de- 
mands of these various classes has been the object of 
the writer in the preparation of this volume. 

As mathematics is simply an abbreviated form of 
language, mathematical reasoning, where required, has 
been reduced, so far as possible, to oidinary language, 
intelligible to unmathematical readers ; thus reducing 
the amount of mathematical formulae required to a very 
few simple expressions, not beyond the capacity of per- 
sons familiar with arithmetic. 

Each chapter is intended to be a complete treatise on 
the subject to which it relates, and the whole thirteen to 
embrace all the essential facts pertaining to that form 
of electricity which manifests itself in currents, as dis- 
tinct from the form known as static, which has been 
similarly treated in the author's " Elements of Static 
Electricity." The publication of the latter book in a 
separate volume, and the elimination of mathematical 
formulae from the present book, have left room for a 
much fuller treatment of the physical principles of elec- 
tric science in this, its most practical form, than would 
otherwise be possible in a volume of this size. 

This book is not intended as an exposition of the 
writer's own peculiar views, but of the well established 
principles of electricity and magnetism, as held by the 
leading electricians of the world. The writer has col- 
lected facts from every available source ; examining 
carefully the views of various eminent writers on each 
point, and giving the net result of such examination in 
his own language ; inspecting the construction and 
practical operation of electric apparatus and machinery, 
and consulting practical, electric experts, familiar with 
such construction and operation. 

The chronological order of electric development has 
been followed, so far as possible, as it is not only in 



IN TR OD UC TION. V 

accordance with the inductive method, which leads from 
first principles to full development, but also furnishes 
a history of electric progress, showing the relations of 
each important invention to those which preceded and 
followed it. 

On important controverted points the leading views 
on both sides have been given; and the selections of 
practical, electric appliances for description have been 
made with strict reference to the adaptation of each to 
illustrate some important principle to the best advan- 
tage. Clearness of style has been recognized as a first 
requisite, and conciseness as subordinate to it. 

The author takes pleasure in acknowledging his obli- 
gations to the following writers : Niaudet, Deschanel, 
Maxwell, Jenkin, Gordon, Thompson, Ayrton, Faraday, 
Fontaine, Gore, Hering, Pope, Gladstone & Tribe, 
Preece & Maier, Maver & Davis, Terry & Finn; also to 
the U. S. Coast and Geodetic Survey for valuable in- 
formation, documents, and magnetic maps; and to the 
following parties for cuts: The E. S. Greeley & Co., 
The W. J. Johnston & Co., Western Electric Company, 
James W. Queen & Co., The Electrical Accumulator 
Company, Thomson Electric Welding Company, The 
Electrical Supply Company, D. Van Nostrand Company, 
A. C. Terry, Weston Electric Instrument Company. 

To the practical electricians of Chicago he is under 
the deepest obligations, for the uniform courtesy shown 
him repeatedly in his search for information, and for 
permission to inspect and test the practical operation of 
apparatus; and he would especially mention, in this con- 
nection, J. K. Pumpelly, Force Bain, Dr. Louis Bell, 
George Cutter, the representatives of the Thomson- 
Houston Electric Company, the Brush Electric Com- 
pany, the Chicago Edison Company, the Westing- 
house Company, the Chicago Arc Light and Power 



VI INTRODUCTION. 

Company, the Electrical Supply Company, Central 
Electric Company, the Electrical Accumulator Com- 
pany, "C & C" Electric Motor Company, the Western 
Union Telegraph Company, the Chicago Bell Telephone 
Company, the American Telephone and Telegraph 
Company. 

Philip Atkinson. 
Chicago, November i, 1890. 



CONTENTS. 



CHAPTER I. 

The Voltaic Battery — Definitions r 

Dynamic Electricity Defined. Discoveries of Galvani. Dis- 
coveries of Volta. The Couronne de Tasses. The Voltaic 
Pile. Value of Volta's Discoveries. Cell, Element, and Bat- 
tery. Battery Sign. Electrodes and Poles. Conditions of 
Electric Energy. Electromotive Force. Resistance. Cur- 
rent. Units of Electromotive Force, Resistance, and Cur- 
rent. Operation of the Voltaic Cell. Theory of Electric 
Generation in the Cell. Amalgamation of the Zinc. Insula- 
tion and Clamping. Polarization. 

CHAPTER n. 
One-Fluid Cells 13 

Smee's Cell. Zinc-Carbon Cells. Walker's Cell. Potas- 
sium Bichromate Cell. The Grenet Cell. The Mercuric 
Bisulphate Cell. The Leclanche Cell. The Law Cell. The 
Diamond-Carbon Cell. Dry Cells. Polarization of One- 
Fluid Cells. 

CHAPTER HI. 

Two-Fluid Cells. Battery Formation 23 

Construction of Two-Fluid Cells. The Daniell Cell. The 
Callaud Cell. The Grove Cell. The Bunsen Cell. The 
Silver Chloride Cell. Battery Formation. Connection be- 
tween Cells. 

CHAPTER IV. 
Magnetism 35 

The Natural Magnet. Magnetic Polarity. The Mariner's 
Compass. The Surveyor's Compass. The Earth's Magnetic 



Vlll CONTENTS. 

Poles. Declination. Inclination or Dip. The Dipping 
Needle. Magnetic Maps. Terrestrial Magnetism Illustrated. 
Magnetic Intensity. Magnetic Force Ascertained by Oscilla- 
tion. Magnetic Force Ascertained by Deflection. Absolute 
Magnetic Intensity. Biot's Law. Origin of Terrestrial Mag- 
netism. Secular Variation. Secular Variation in the United 
States. Annual and Diurnal Variation. The Eleven Year 
Period. Magnetic Storms. Cosmic Variation. Exact Ob- 
servation. Secular Variation at Washington. Secular Varia- 
tion at San Francisco. Artificial Magnets. Magnetic Satu- 
ration. The Armature. Laminated Magnets. Magnetic 
Loss. Portative Force. Polar Attraction and Repulsion. 
Magnetic Lines of Force. Magnetic Field. Form of Mag- 
nets. Magnetic Penetration. Location of the Poles. Para- 
magnetic and Diamagnetic Bodies. Magneto-Crystallic In- 
duction. Magnetism as a Mode of Molecular Motion. Anal- 
ogy between Magnetic and Electric Phenomena. Coulomb's 
Torsion Balance. The Gauss- Weber Portable Magnetometer. 

CHAPTER V. 

Electromagnetism. 71 

Deflection by th-. Electric Current. The Galvanoscope. 
The Schweigger Multiplier. Ampere's Rule. The Astatic 
Needle. Compensating Magnet. Cause of Deflection. The 
Electromagnet. Electromagnetic Poles. Winding. Mag- 
netic Strength. Core. Helix Coefficient of Magnetic In- 
duction. Electromagnetic Saturation. Form of Electro- 
magnets. Armature. Experiments in Diamagnetism. List 
of Diamagnetic and Paramagnetic Substances. Deflection of 
the Electric Current by the Magnet. Ampere's Table. 
The Solenoid. De La Rive's Floating Battery. Mutual In- 
duction of Electric Currents. Rotary Movement by Current 
Induction. Ampere's Theory of Magnetism. Generation of 
Electric Currents by Induction. Current, Induced by Magnet. 
Current Induced by Another Current. Current Induced by 
Opening or Closing Primary Circuit. Current Induced by 
Varying the Strength of Primary Circuit. Results of Current 
Induction. Generation of Current Dependent on Variation 
of Intercepted Magnetic Force. Coefficient of Mutual In- 
duction. Self-induction. Extra Current. The Spark. . In- 



CONTENTS. IX 

duction of Core. Induction Coil. Condenser. Interrupter. 
Sliding Core. Water Rheostat. Construction of Core. 
Operation of Condenser. Leyden Jar as a Condenser. 
Special Construction. Ruhmkorff's Commutator. The Coil 
a Converter. Electric Perforation. Physiological Effects of 
Faradic Current. Discharge in Air and in Vacuo. Electric 
Gas Lighting. Spark Coil. 

CHAPTER VI. 
Electric Measurement no 

Electric Potential. Electromotive Force. Electric Resist- 
ance. Insulation and Conductivity. Electric Current. Ohm's 
Law. Electric Units. The Volt. The Mircrovolt. The 
Ohm. The Megohm. The Ampere. The Milliampere. 
The Ampere- Hour. The Coulomb. The Farad. The 
Microfarad. The Watt. The Electric Horse- Power. Dif- 
ferent kinds of Electric Measurement. Electrometers. Gal- 
vanometers. Measurement of Angles. Angular Measure- 
ment of Deflective Force. Calibration of Galvanometer. 
Sine Galvanometer, Tangent Galvanometer. Astatic Gal- 
vanometer. Thomson's Reflecting Galvanometer. Differen- 
tial Galvanometer. Ballistic Galvanometer. Common Gal- 
vanometers. Voltmeters and Ammeters. The Weston Volt- 
meter. The Weston Ammeter. The Weston Milliammeter. 
The Wirt Voltmeter. Ayrton and Perry's Spring Voltmeters 
and Ammeters. Gravity Ammeters. The Cardew Volt- 
meter. The Edison Current-Meter. The Forbes Coulomb- 
Meter. Voltameters. The Water Voltameter. The Weber- 
Edelmann Electrodynamometer. Measurement of Electric 
Resistance. Resistance Coils. The Wheatstone Bridge. 

CHAPTER VII. 
The Dynamo and Motor 165 

The Magneto-Electric Generator. Commutation. The 
Alliance Machine. The Siemens Armature. Wilde's Ma- 
chine. The Dynamo. Ladd's Machine. The Pacinotti- 
Gramme Armature. Improved Commutator. Direction of 
Current. Interior Wire of the Gramme Armature. The 
Cylinder Armature. Closed-Circuit and Open-Circuit Arma- 
tures. Location of the Armature's Magnetic Poles. Mag- 



X CONTENTS. 

netic Lag. Position of the Brushes. The Field- Magnets. 
Series, Shunt, and Compound Winding. Constant Current 
Dynamo. Constant Potential Dynamo. The Edison Dyn- 
amo. Alternating Current Dynamos. The Gordon Dynamo. 
The Westinghouse Dynamo. Separate Excitation. Advan- 
tages of the Alternating Current Dynamo. The Converter. 
Development of the Electric Motor. The Dynamo as a Mo- 
tor. Principles of the Motor. Loss of Energy. Eddy Cur- 
rents. Series, Shunt, and Compound Wound Motors. Re- 
versible Rotation. The Alternating Current Motor. The 
Westinghouse Tesla Motor. The Tesla Motor as a Converter. 
Reversal of Rotation. Distribution of Power. Elevated- 
Road Distribution. Thermo-Magnetic Motors. 

CHAPTER Vin. 

Electrolysis 206 

Nomenclature by Faraday. Theory of Grotthus. Elec- 
trolysis of Water. Conditions of Electrolysis. Secondary 
Reaction. Electrolysis of Mixed Compounds. Relations of 
Electrolysis to Heat. Lowest Required Electromotive Force. 
Faraday's Laws. Magnetic Effects. Chemical Equivalence. 
Electrochemical Equivalence. Effect of Current Reversal. 
Effect of Convection. Relative Conditions of Current and 
Electrolyte. Electroplating. Various Details. The Anodes. 
Plating Solutions. Auxiliary Operations. Required Elec- 
tric Energy. Required Time of Immersion and Thickness of 
Deposit. Agitation of the Solution. Electrotyping. Elec- 
tric Refining of Metals. Electric Reduction of Ores. The 
Hall Process for Aluminium. 

CHAPTER IX. 

Electric Storage 233 

The Leyden Jar and Condenser. Grove's Gas Battery. 
Plante's Secondary Cell. Chemical Reaction. The Faure 
Cell. Chemical Reaction. Defects of the Faure Cell. Im- 
proved Faure Cell. Electric Preparation of the Plates. Elec- 
tric Energy of Improved Cell. Effects of Charge and Dis- 
charge on the Plates. E. M. F. of discharge. Conductivity 
and Buckling. W^eight of Cells. Composition of Grids. 



CONTEXTS. XI 

The Julien Cell. The Pumpelly Cell. Durability of Storage 
Cells. Storage Capacity. Relative Time of Charging and 
Discharging. The Hydrogen Alloy Theory. 

CHAPTER X. 

The Relations of Electricity to Heat 252 

Heat Developed by Electric Transmission. Joule's Law. 
Joule's Equivalent. Heat Developed by Electrochemical 
Action. Electro-Thermal Capacity of Conductors. Electric 
Blasting. Electric Cautery. Electric Fuses. Thermo-Elec- 
tric Generation. Thermo-Electric Diagrams. The Peltier 
Effect. Thermo-Electric Inversion. The Thomson Effect. 
The Thermopile. Electric Welding. 

CHAPTER XI. 

The Relations of Electricity to Light 279* 

The Relations of Electric Heat to Electric Light. Photo- 
Electric Generation. Photo-Electric Reduction of Resistance 
in Selenium. Polarization of Light. Magneto-Optic Polari- 
zation — Faraday's Discoveries. Verdet's Discoveries. Bec- 
querel's Discoveries. Kiindt and Rontgen's Discoveries. 
Kerr's Discoveries. Effects of Double Reflection. Summary. 
Maxwell's Theory. Molecular Theory. Strain in the Media. 
Electric Lighting. The Arc Light. The Arc. Electric 
Candles. The Arc Lamp. The Crater and Point. The 
Heat and Light. Establishment of the Current. The 
Carbons. Automatic Regulation. Hefner von Alteneck's 
Regulator. Series Distribution. Automatic Cut-Out. The 
Incandescent Lamp. The Filament. Filament and Lamp 
Attachment. Position and Current. Parallel Distribution. 
Multiple Series and Series Multiple. Three- Wire System. 

CHAPTER XII. 

The Electric Telegraph. . , 310 

Early History. The American Morse Code. The Inter- 
national Morse Code. Simple Line Equipment. The Bat- 
tery. The Key. The Register. The Sounder. The Relay. 
Cut-Out, Ground-Switch, and Lightning-Arrester. Line 



XU CONTENTS. 

Construction. Station Arrangement. Switch-Board. Re- 
peaters. The Button Repeater. The Milliken Repeater. 
Repeater Connections. Duplex Telegraphy. The Stearns 
Duplex. The Polar Duplex. The Pole-Changer. The Pola- 
arized Relay. Operation of the Polar Duplex. Quadruplex 
Telegraphy. Construction and Operation of the Quadruplex. 
Repeating by the Quadruplex. Substitution of the Dynamo 
for the Battery. The Wheatstone System of Automatic 
Rapid Transmission. Submarine Telegraphs. Locating 
Faults. The Dial Telegraph. Printing Telegraphs. 

CHAPTER XIII. 

The Telephone 359 

Early History. Principles of the Telephone. The Bell 
Telephone. Improved Transmitters. The Edison Trans- 
mitter. The Blake Transmitter, Accessory Apparatus. 
The Signaling Apparatus. The Exchange. The Multiple 
Switch-Board. Hughes' Microphone. Theory of Telephonic 
Transmission. Multiplex Telephony. Long Distance Tele- 
phony. Van Rysselberghe's System. The American System. 
The Hunning Transmitter. Transmission on Long Distance 
Lines. 



I 



THE ELEMENTS OF DYNAMIC ELECTRI- 
CITY AND MAGNETISM. 



CHAPTER I. 

THE VOLTAIC BATTERY. DEFINITIONS. 

Dynamic Electricity Defined. — The term dynaf?tic, from 
dvva/AiS, power, is appropriately used to designate elec- 
tricity when employed for useful work, embracing the 
electric phenomena pertaining to that state of electric 
motion termed current^ by which apparatus or machinery 
is operated, as distinct from that class of phenomena 
termed static^ which pertains chiefly to electricity when 
stationary and not employed in this way. Hence it may 
be accepted as properly including all the various electric 
phenomena to which the ^t.r^ix^^ galvanic^ voltaic^ current^ 
chemical^ magneto^ and thermo have been applied. 

Discoveries of Galvani. — In 1780, Galvani, a professor 
of anatomy at Bologna, Italy, observed certain muscular 
contractions in the limbs of frogs recently killed, pro- 
duced by electricity generated by a frictional machine. 
He subsequently noticed similar contractions when tiie 
frogs' limbs were hung on an iron balcony by copper 
hooks in contact with the lumbar nerves. Placing a 
pair of them on an iron plate, and touching the lumbal 
nerves with a copper wire the opposite end of which 

I 



2 DYNAMIC ELECTRICITY AND MAGNETISM. 

was in contact with the plate, he reproduced the mus- 
cular movements. From this he inferred that the 
nerves and muscles were oppositely electrified, and that 
the muscular action was due to the establishment 6f a 
connection between them. 

Discoveries of Volta. — Volta, a professor of physics at 
Pavia, Italy, having observed that the movements were 
produced by using a muscle in connection with tv/o 
metals, inferred that they were due to the electricity 
generated by the contact of the metals when the damp 
muscle was placed between them, and that if the same 
conditions were produced in some other way, electric 
generation would follow. On this hypothesis he con- 
structed, in 1800, the apparatus known as the coiironne 
de tasses^ or crown of cups. 

The Couronne de Tasses. — This apparatus consisted of 
a series of cups or glasses, arranged in a circle, each 
containing a zinc plate and a copper plate partly im- 
mersed in a solution of salt in water, the copper of 
each cup being joined by a copper conductor to the 
zinc of the next cup, the fluid intervening between the 
two metals. Connection being made by a conductor 
between the copper of the first cup and the zinc of the 
last, strong electric effects were obtained, and the dis- 
covery excited great interest in the scientific world, as 
friction was the only means previously known of gener- 
ating electricity. 

The Voltaic Pile. — Volta subsequently invented a por- 
table apparatus, intended for medical, electric treat- 
ment in ho'pltals, known as the voltaic pile. This ap- 
paratus consisted of a series of copper and zinc disks, 
arranged in a pile, with disks of cloth, moistened with 
a solution of salt in water, between each pair ; the low- 
est disk being copper, the next zinc, and the next cloth 
the same order being continued throughout the pile, s( 



THE VOLTAIC BATTERY. DEFIXITIONS. 3 

that the topmost disk was zinc. Connection being made 
between the top and bottom disks, as between the ter- 
minal plates of the couronne de tasses, similar electric 
effects were obtained. 

This apparatus was also constructed with copper and 
silver coins. Water acidulated with sulphuric acid was 
also used instead of the solution of salt in water, both 
for the pile and the couronne de tasses. 

Value of Volta's Discoveries. — These discoveries laid the 
foundation of the science of dynamic electricity, and 
Volta's apparatus is the type of all the batteries since 
constructed. The value and importance of his work 
become apparent when we consider that after nearly a 
century of constant experiment by eminent scientists the 
metals he employed are still found to be the most efficient 
and economical for this purpose, while his arrangement 
of the elements in series is still found to be the arrange- 
ment which produces the highest electric potential. 
The use of zinc in battery construction has never been 
superseded. It has been employed in nearly every bat- 
tery that has ever been invented, and enters into the 
construction of every one now in general use. And 
copper, in connecction with it, is the next metal in most 
general use for this purpose. 

Cell, Element, and Battery. — A single pair of metals or 
their equivalent, with the fluid and containing vessel, 
or substance, is designated as a cell or ele?ne?it, and a 
combination of such cells is called a battery j the latter 
term being also applied to a single cell, when employed 
alone. 

Battery Sign. — This sign, |i| i|i, is used to represent 
the battery; the short, heavy lines representing the 
zinc, and the light ones the copper or its equivalent; 
the number of lines varying indefinitely, according to 
the size of the battery, each pair representing a cell. 



4 DYNAMIC ELECTRICITY AND MAGNETISM. 

Electrodes and Poles. — Since the metals, or their equiv- 
alents, are the principal avenues in which the electricity 
travels, downward through the zinc and upward through 
the copper, or its equivalent, they are called the elec- 
trodes^ from r}\eKrf>oy oSo^, electric road. The zinc, 
being consumed by the chemical reaction, is termed the 
soluble or generating electrode, and the copper the con- 
ducting electrode. This term is also applied to instru- 
ments used for conveying and applying electricity. 

The parts of the electrodes which project out of the 
fluid are known as the poles; the projecting part of the 
zinc being designated as the negative pole, and that of 
the copper, or its equivalent, as the. positive pole. These 
terms are also applied respectively to the outer ter- 
minals of the conducting wires connected with the poles 
of a battery or other electric generator. 

The terms positive and negative are also applied to 
the eleotrodes, the zinc being called the negative elec- 
trode, and the copper, or its equivalent, th^positive. 

Conditions of Electric Energy. — In estimating the elec- 
tric energy of a cell three important conditions are to 
be considered, termed respectively electromotive force^ 
resistance^ and current ; any two of which being known, 
the third can be ascertained by calculation. 

Electromotive Force. — Electromotive force, symbol 
E. M. F., has been defined as "that which moves or 
tends to move electricity from one point to another." It 
is represented by difference of electric potential; elec- 
tricity always moving, or tending to move, from higher 
to lower potential with a force, or pressure, equal to 
this difference. This condition, in the cell, depends on 
the nature of the materials employed and their mutual 
relations, varying in proportion to the chemical reaction 
between the soluble electrode and fluid, and the resist- 
ance to such reaction and the electric, molecular move- 



THE VOLTAIC BATTERY. DEFINITIONS. 5 

ment generated by it, by the various materials com- 
posing the cell. Hence this is not properly a force, but 
a condition producing force. 

Resistance. — Resistance, symbol R, is that which opposes 
the movement of telectricity through a conductor; and, 
in the cell, it depends chiefly on the nature of the fluid, 
the quantity intervening between the electrodes, and a 
certain effect termed polarization. It varies directly as 
the length and inversely as the cross-section of the con- 
ductor; and since the distance between the electrodes 
may be regarded as the length of the fluid conductor, 
while the area of their immersed surfaces measures its 
cross-section, the fluid resistance of a cell varies directly 
as the distance between the electrodes, and inversely 
as the area of their immersed surfaces; hence the least 
resistance, dependent on these conditions, is obtained 
with the shortest practicable distance between the elec- 
trodes coupled with the greatest area of immersed sur- 
face. 

The resistance of battery fluids varies greatly; that 
of pure water or acid alone, for instance, is very high, 
but in mixtures of the two the resistance is greatly re- 
duced. Hence the importance of selecting the fluid 
with reference to its resistance as well as its chemical 
reaction. 

Current. — Current, symbol C, is the electric movement 
produced in a conductor by electromotive force in 
opposition to resistance; its value being ascertained by 
dividing the former by the latter. Hence strength of 
current varies as each of these factors, increasing 
with increase of E. M. F. or decrease of resistance, and 
decreasing with decrease of E. M. F. or increase of re- 
sistance, but remaining constant when each varies in 
the same ratio as the other. 



6 DYNAMIC ELECTRICITY AND MAGNETISM. 

Units of Electromotive Force, Resistance and Current.— 

The Volt is the unit of electromotive force, repre- 
sented practically by the E. M. F. of the Daniell cell, 
to be described hereafter, to which it is nearly equal. 

The Ohm is the unit of electric resistance, represented 
by the electric resistance of a column of mercury 106 
centimeters in vertical height, and i square millimeter 
in cross-section, at the temperature of 0° C. 

The Ampere is the unit of current strength, repre- 
sented by an E. M. F. of i volt divided by a resistance 
of I ohm. 

As electric measurement pertains to a future chapter 
in which it is resumed and treated at greater length 
the above brief definitions of the three principal elec- 
tric units must suffice for our present purpose. 

Operation of the Voltaic Cell. — If the metals are strictly 
pure there is no perceptible action either chemical or 
electric in the voltaic cell so long as there is no connec- 
tion between the electrodes ; but when the poles are 
brought into contact, or connected by a conductor, 
chemical reaction, accompanied by the generation of 
electricity, begins at once. If the metals are impure, as 
is usually the case, chemical reaction and electric gen- 
eration, in a limited degree, occur without polar connec- 
tion. In either case the water is decomposed, the hy- 
drogen collecting on the surface of the copper, and the 
oxygen combining with the zinc, forming oxide of zinc, 
which then combines with the sulphuric acid, forming 
sulphate of zinc. The generation of electricity may be 
proved by separating the poles slightly, when an electric 
spark will pass between them. 

Theory of Electric Generation in the Cell. — Volta, as we 
have seen, attributed the electric generation to the con- 
tact of the metals, and this was the accepted theory 
among scientific observers to the time of Faraday. 



THE VOLTAIC BATTERY. DEFINITIONS. 7 

Meantime chemistry, almost unknown as a science in 
Volta's time, had made rapid advancement, and Fara- 
day s observations having led him to the conclusion 
that the mere contact of the metals was not an adequate 
cause for the results obtained, and was not proportionate 
to such results, made an investigation of the relations 
between the chemical and electric actions of the cell, 
which enabled him to demonstrate that the electric 
generation was in exact proportion to the chemical 
reaction ; and his results having been fully verified by 
other observers, the chemical theory of electric genera- 
tion in the cell has since been generally accepted as cor- 
rect. It may be briefly stated as follows : 

The principal seat of chemical reaction is at the sur- 
face of the zinc, which is consumed by oxidation, while 
the copper acts as a conductor and is not consumed. 
Hence, since electric movement is from higher to lower 
potential, and the same law applies to the energy of 
chemical reaction, in common with other forms of 
physical energy, and since the electric energy of the 
cell is found to be strictly proportionate to its chemical 
reaction, it is assumed that the electric current origi- 
nates at the surface of the zinc and flows through the 
fluid to the copper. 

In the absence of external connection between the 
metals, it is evident that the difference of electric poten- 
tial would immediately become equalized and the cur- 
rent cease, but when they are brought into external 
contact, or connected by a conductor, the current finds 
an outlet through the copper, and flows back to the 
zinc through the external circuit ; chemical reaction is 
thus sustained and the current becomes continuous. 

The electric generation produced by Zambont'' s dry pile 
is adduced in proof of the contact theory. This pile 
was made of a large number of paper disks^ some thou- 



8 DYNAMIC ELECTRICITY AND MAGNETISM. 

sands, coated with zinc or tin foil on one surface, and 
with dioxide of manganese on the other, and closely 
compressed in a glass tube, their similarly coated sur- 
faces turned in the same direction, bringing those op- 
positely coated into contact. Such a pile, when its cir- 
cuit is completed, as in Volta's pile, can excite the 
electroscope, ring a bell, or give sparks. But this elec- 
tric action can be accounted for by chemical reaction, 
caused by dampness in the paper, rather than by the 
mere contact of different substances. 

Such experiments as the divergence of the leaves of 
the electroscope and the oscillations of the magnetic 
needle by the mere contact of different metals in their 
immediate vicinity are also adduced in support of the 
contact theory; but such electric action is doubtless due 
to the static charge generated by the slight friction 
produced in making the contacts. 

The law of the conservation of energy requires the 
expenditure of energy in one form as a condition of the 
production of the same amount in another form. Now 
in every electric generator, static or dynamic, machine 
or battery, this law is found to be strictly true ; there 
must be a complete circuit of materials differing in mo- 
lecular constitution, and the expenditure of energy, 
mechanical, chemical, or in some other form, at some 
point in the circuit as a condition of electric generation; 
and this expenditure must be equal in amount to the 
electric energy produced and that absorbed by friction, 
heat, or otherwise. Hence as chemical energy is the 
only energy expended in the battery, the conclusion is 
inevitable that it is the source of the electric energy 
generated. 

Amalgamation of the Zinc. — As strictly pure zinc is too 
expensive for practical use in battery cells, and ordinary 
commercial zinc contains a certain percentage of iron 



THE VOLTAIC BATTERY. DEFINITIONS 9 

and other metals by which chemical and electric action 
.s generated independent of the copper, and the energy 
thus, in part, expended within the cell, without passing 
through the external part of the circuit, where it can be 
made available, a fault known as local action^ the method 
has been adopted of amalgamating the surface of the 
zinc with mercury, which renders it more homogeneous 
and prevents any serious interference from local action, 
which is thus reduced to its minimum. 

The zinc is first cleansed with potash or otherwise, 
after which the mercury, mixed with acid, is applied by 
any convenient method, or the zinc dipped into the 
mixture. Sulphuric acid may be used for this purpose, 
but a mixture of five parts chlorhydric and one part 
nitric acid is preferable. The same result is also ob- 
tained by adding bisulphate of mercury to the solution, 
the mercury combining with the zinc, and the acid being 
set free. Amalgamation is thus more easily accomplished 
and better sustained. 

The molten zinc, before it is cast into plates, may be 
permanently amalgamated by the addition of about 4 
per cent of mercury, and thus the frequent renewal, 
necessary with surface amalgamation, be dispensed 
with. 

Insulation and Clamping. — When both electrodes are 
suspended from the support, they must be insulated 
from each other, either by making the support of insu- 
lating material, or insulating one of them from it. They 
must also be provided with clamps and binding-screws 
for making connections, and the points of contact with 
conductors kept clean and free from oxidation. 

Polarization. — It has been stated that, as a result of 
the chemical reaction of the cell, hydrogen accumulates 
on the surface of the copper^ As this accumulation in- 
creases, it weakens the electric action and finally stops 



10 DYNAMIC ELECTRICITY AND MAGNETISM. 

it ; an effect \.^xvci^6. polarization . As a thorough knowl- 
edge of this effect and the methods used to correct it is 
of the highest importance in the study of the cell, it is 
proper first to examine its nature. 

If the poles of a battery of two or more cells be con- 
nected with platinum terminals which project into a 
vessel of water acidulated with sulphuric acid, hydrogen 
will be evolved at the terminal connected with the zinc, 
or negative pole, and oxygen at that connected with the 
copper, or positive pole, in the exact proportions which 
form water, two volumes of hydrogen and one of oxygen. 
If now the battery be disconnected, and the terminals 
of the wires connected with the gas tubes brought into 
contact, an electric current will flow through them in 
the reverse order to that of the original current, the 
gases, at the same time, recombining to form water. 
From which it is evident that the electric energy ex- 
pended in decomposing the water was stored up in the 
gases, and reappears when they return to their original 
state. 

Fig. I shows the apparatus by which this decomposi- 
tion is effected ; oxygen being evolved in the right-hand 
tube and hydrogen in the left. It will be noticed that 
the hydrogen is evolved at the pole towards which the 
current flows within the decomposing vessel, connected 
externally with the zinc of the battery, and the oxygen 
at the pole from which it flows, connected externally 
with the copper of the battery ; the external current 
through the wires connecting with the battery being 
towards the oxygen tube and from the hydrogen tube ; 
also that the same direction of current-flow occurs with 
respect to the battery, internally from zinc to copper, 
externally from copper to zinc, completing the circuit 
through the decomposing vessel. And since the current 
from the gas tubes, when disconnected from the battery 



THE VOLTAIC BATTERY. DEFINITIONS. 



ir 



and brought into mutual contact, flows in the reverse 
order to that of the original battery current, it is evi- 
dent that when the gases accumulate on the electrodes 
within the cell, the effect must be to set up a similar re- 




FlG. I. 

verse current, which neutralizes the primary current. 
Hence this action is appropriately termed polarization, 
since it produces opposing poles. 

But since the oxygen, from its strong affinity for the 
base metals, combines with the zinc, the polarization is 
confined to the hydrogen, taking place on the copper. 
This affinity of the oxygen makes the use of a platinum 
terminal necessary for the oxygen at least, when it is 
desired to collect the gases separately, as above, since 
oxygen does not combine with platinum; while, if a 
base metal were used, it would become oxidized, and 
no oxygen gas could be collected. 

A single cell of less E. M. F. than T.49I volts is insuf 
ficient to decompose water, since the polarizing energy, 
in such case, exceeds the generating energy; hence two 
such cells at least are required. 



/2 DYNAMIC ELECTRICITY AMD MAGNETISM. 

To correct polarization the accumulation of the hy- 
drogen must be suppressed, and to do this in the most 
effectual, practical, and economical way, without impair- 
ing the energy of the cell in other respects, is the most 
important problem in cell construction. It may be done 
either by mechanical or chemical means, the latter be- 
ing the most practical and effectual. Among the me- 
chanical means adopted are the lifting of the electrodes, 
or the conducting electrode alone, out of the fluid, so 
that the hydrogen may pass off. With a battery of two 
or more cells the electrodes of half the cells may thus be 
depolarized while the other half remain in the fluid 
and furnish the current. And as only a momentary ex- 
posure is required, any simple mechanism, operated by 
a weight or spring, by which this alternate exposure can 
be effected will answer the purpose. Another method 
is the injection of air into the fluid against the conduct- 
ing electrode. Either of these methods may be em- 
ployed for work which does not require a continuous, 
strong current ; and they are sometimes used in connec- 
tion with the chemical process to intensify the electric 
action. But all mechanical contrivances for this pur- 
pose are necessarily cumbersome and inconvenient, and 
hence undesirable. 

The chemical method is to introduce into the cell 
some substance whicfi has a strong chemical affinity for 
the hydrogen, and -absorbs it without interfering with 
the action of the cell in other respects. This is accom- 
plished either by the use of a single fluid holding the 
substance in solution, or by using two fluids separated 
by a porous cup or otherwise, so that the zinc shall be 
in contact with one fluid, and the copper, or its equiva- 
lent, in contact with the other. Hence arises the divis- 
ion of cells into two classes, o?ie-f[uid and fwo-Hmd cells, 
each of which now claims our attention. 



ONE-FLUID CELLS. 



\% 



CHAPTER IL 



ONE-FLUID CELLS. 



Smee's Cell. — This cell, represented by Fig 2, was in- 
vented by Smee, an English electrician, in 1840. The 
electrodes consisted originally of a plate of platinum 
suspended between two plates of zinc ; the object of 
this arrangement being to utilize both surfaces of the 
platinum, since, in any cell, only the ad- 
jacent surfaces of the opposite electrodes 
are brought into action. Depolariza- 
tion was effected by platinizing the 
surface of the platinum, electrically, so 
as to furnish a rough surface from 
which the hydrogen could escape much 
more freely than from a smooth surface, 
since a point has neither adhesion nor 
electric resistance, and the hydrogen 
atoms, being at the same electric poten- 
tial, are self-repellent. 

The platinum plate was subsequently replaced by a 
platinized silver plate, and this was afterwards replaced 
by a copper plate, covered with a rough coating of cop- 
per, then silver-plated and then platinized. The fluid 
consists of one part sulphuric acid to seven parts water. 

This cell is practical and efficient, where constancy of 
current is not required ; but, like all single-fluid cells, 
the current soon weakens. 

Zinc-Carbon Cells. — The expense of constructing cells 
with platinum or silver stimulated the search for some 




Fig. 



14 DYNAMIC ELECTRICITY AND MAGNETISM. 

cheaper material, and Sir William Grove first suggested 
the use of carbon, but failed to reduce his suggestion to 
practice. In 1843 Bunsen constructed the first cell in 
which carbon was used. This was a two-fluid cell, and 
will be described under that head. Since that time 
carbon has been successfully employed in the construc- 
tion of numerous different cells which have come into 
general use. 

Carbon suitable for this purpose may be obtained 
from the inside of gas-retorts, and cut into plates or other 
convenient forms. It may also be prepared from coal, 
coke, graphite, or charcoal, pulverized, cemented to- 
gether, reduced to the proper form in moulds ; then 
dried, baked, and soaked in sirup of sugar repeatedly, 
till it acquires the requisite density and firmness. 

To obtain a good connection for the clamps and con- 
ducting wires, it is desirable that the upper part of the 
carbon should be soaked in melted paraffine, and then 
copper-plated. The paraffine fills the pores, and ex- 
cludes the acid, which would otherwise ascend by capil- 
lary attraction and destroy the copper. 

The advantages of carbon are: i. That it is cheap. 
2. That, like platinum, it is insoluble in acid, and pos- 
sesses the conductivity necessary for an electrode. 3. 
That it has a rough surface, similar to that produced 
artificially with platinum in Smee's cell, by which de- 
polarization is assisted. 4. That, being porous, a great 
amount of internal surface is brought into contact with 
the fluid. 

Walker's Cell. — Walker was one of the first to use car- 
bon as an electrode. In 1849 he constructed a cell sim- 
ilar to the Voltaic, substituting carbon for copper. In 
1857 he platinized the carbon, copper-plated and tinned 
its upper end, placed the lower end of the zinc in a ves- 
sel of mercury, by which it was kept amalgamated; and 



ONE- FLU ID CELLS. 1 5 

used a fluid composed of one part, by volume, of sul- 
phuric acid to eight parts of water. 

This cell has great constancy, requires but little care, 
and is cheaply constructed. Its electric energy is about 
the same as that of the Smee cell. It has been exten- 
sively used in England for telegraphing, vi^ith great 
success. 

Potassium Bichromate Cell.— The most efficient single- 
fluid, carbon and zinc cell is that in which potassium 
bichromate is the depolarizing agent. The fluid con- 
sists of water, sulphuric acid, and potassium bichro- 
mate, and the following are recommended as the best 
proportions : 

(i(i per cent by weight of water, 
25 " " " " " sulphuric acid, 
9 *^ " " " " potassium bichromate. 

The bichromate is decomposed by the sulphuric acid, 
and oxygen liberated, which enters into combination 
with the hydrogen while both are in the nascent state, 
producing water, and thus preventing the accumulation 
of the hydrogen. Practically, however, there is a cer- 
tain amount of polarization, and salts are also formed, 
which, if allowed to accumulate, reduce the conductivity 
of the carbon; so that the intensity of the electric action 
soon diminishes, and the fluid requires to be agitated, 
either by injecting air into it, or by withdrawing the 
electrodes, or the zinc alone. Air may be injected 
through a rubber tube; but this method, though very 
effective, is inconvenient in practice, and the withdrawal 
of the electrodes is preferable. Hence this cell is best 
adapted to work where constancy is not required; so 
that after a few minutes' use the electrodes may be 
withdrawn and the cell allowed to recuperate, while 
preparation is made for the next operation. Medica. 
surgical, and laboratory work is of this character, and 
for such work it is especially fitted; having the highest 



:6 DYNAMIC ELECTRICITY AND MAGNETISM. 



electric energy of any single-fluid cell in use ; being 
capable of application to a great variety of different 
operations; being free from noxious fumes; and easily 
made portable, either as a single cell, or a battery of 
cells. 

It is usually fitted with a hard-rubber cover, to which 
the electrodes are attached, and thus insulated. And as 
depolarization is in proportion to the relative amount 
of surface of the conducting electrode brought into 
action as compared with that of the soluble electrode, 
it is usual to have a carbon plate on each side of the 
zinc plate; using two carbons and one zinc, or three 
carbons and two zincs. 

As this fluid soon weakens with use, and is subject 
to slow chemical change when not in use, the amount 
should be so proportioned to the size of the electrodes 
as to prevent rapid exhaustion. 

The Grenet Cell. — The bottle form of the bichromate 
cell, known as the Grenet, shown in Fig. 3, is conven- 
ient for work requiring only a sin- 
gle cell. The electrodes are at- 
tached to a close-fitting hard-rub- 
ber cover, and the zinc is connected 
with a sliding rod by which it can 
be drawn up into the wide neck, 
while the enlarged base gives the 
requisite capacity for a full supply 
of fluid. 

The zinc of any bichromate cell 
should be kept well amalgamated, 
and when the fluid is renewed, the 
deposit of chrome alum which ac- 
cumulates in the bottom of the 
vessel should be removed, and the 
Fig. 3. carbons soaked in warm water to 

remove similar deposits from their pores. 




ONE- FLU ID CELLS. 



7 



The Mercuric Bisulphate Cell. — This cell is extensively 
used for medical pocket-batteries, which are usually 
constructed with two small zinc and carbon cells, each 
about an inch square and half an inch deep. The car- 
bon is placed in the bottom of a hard-rubber cup, and 
the zinc, resting on a ledge which insulates it, forms the 
cover. 

The fluid consists of a solution of mercuric bisul- 
phate in water; a few grains of the bisulphate to a tea- 
spoonful of water being sufficient for a cell. The acid 
of the bisulphate unites with the zinc, setting the mer- 
cury free, which keeps the zinc amalgamated. 

The solution can be made up quickly, and renewed 
when wanted; and the cell is 
easily cleaned and requires 
but little care. 

The Leclaiich6 Cell. — Lec- 
lanche, a French electrician, 
was the first to use sal-ammo- 
niac (NH^Cl) intheconstruc- 
tion of battery cells. Fig. 4 
represents this cell, which 
consists of a glass jar, in the 
centre of which is placed a 
porous cup containing a 
carbon plate, which projects 
above it as shown, and is 
surrounded with crushed 
carbon and crystals of man- 
ganese binoxide, mixed in 
about equal proportions. 
This cup is closed with Port- 
land cement, except two 
small openings left for ventilation, and its contents con- 
stitute the conducting electrode. The zinc is a round 




Fig. 



1 8 DYNAMIC ELECTRICITY AND MAGNETISM. 

rod, about half an inch in diameter, placed in a recess 
provided for it in the outer vessel. 

The fluid is a saturated solution of sal-ammoniac in 
virater ; about 6 oz. of the salt being required for a 
quart cell, v^rhich is kept about two thirds full, and its 
upper surface coated with paraffine, to prevent surface 
accumulation of the salt. This solution permeates the 
porous cup and materials contained in it, a little water 
being added through the ventilating openings. 

The manganese binoxide being rich in oxygen, which 
is evolved by the chemical action, acts as a depolarizer, 
the oxygen uniting with the hydrogen to form water ; 
the large proportion of surface in this electrode to that 
of the zinc greatly facilitating depolarization. But if 
electric action is continued too long at a time, an excess 
of hydrogen accumulates, oxygen not being generated 
with sufficient rapidity to unite with it, and polarization 
ensues, requiring a period of rest for the absorption of 
the hydrogen. Hence it is not fitted for work on a con- 
tinuously closed circuit. 

The E. M. F. of this cell is about 1.48 volts, and its 
resistance comparatively low, being reduced by the im- 
proved conductivity of the mixture constituting the 
conducting electrode. The current is always in full 
proportion to the consumption of material, there being 
no chemical action except with a closed external cir- 
cuit, and hence no waste of material by local action or 
otherwise. The electrodes can therefore remain per- 
manently immersed in the fluid without detriment, so 
that the cell can remain undisturbed till the fluid is ex- 
hausted, a little water being added occasionally to sup- 
ply the loss by evaporation. It contains no poisonous 
materials, emits no noxious fumes, and can endure a 
temperature of — 16° C. without freezing or decrease of 
electric energy. 



ONE- FLU ID CELLS. 



19 



The above style of Leclanche cell is known as the 
Disque. In a more recent style, known as the Frism or 
Gotida, the porous cup is dis- 
pensed with, and the conduct- 
ing electrode constructed with 
two prisms, attached by stout 
rubber bands to the central 
carbon plate as shown in Fig. 
5 ; spaces for the circulation 
of the fluid being left between 
the plate and prisms. 

These prisms are composed 
of the double chloride of iron 
and ammonia, mixed either 
with manganese binoxide, 
graphite, or powdered retort- 
carbon, as preferred, and ce- 
mented together with any suit- 
able glutinous substance, as 
tar, rosin, or gum-lac. The 
greater compactness of this 
form of electrode gives it ^^^- 5- 

higher conductivity than the Disque form, while the 
suppression of the porous cup reduces the resistance. 
The electrodes are suspended from a close-fitting cover 
of insulating material b}'' enlarged pole-pieces, which 
close the openings through which they pass. 

The Leclanche cell, in both styles, has come into ex- 
tensive use for open circuit work in which there are con- 
tinually recurring intervals of rest ; and in France it is 
used for telegraphing, to which it is found to be well 
adapted in offices where the work required is not so 
constant as to cause inconvenience from polarization ; 
cells having been used for nine years without renewal 
of the zincs, and only one renewal of the sal-ammoniac 




20 DYNAMIC ELECTRICITY AND MAGNETISM 



The success of the Leclanche has given rise to a num- 
ber of similar cells ; carbon and manganese binoxide, 
variously combined, being employed as the conducting 
electrode ; the soluble electrode in all of them being a 
zinc rod, as in the Lelanche. 

The Law Cell. — Prominent among these is the Law 
cell, the electrodes of which are shown in Fig. 6. The 
conducting electrode consists 
of two hollow cylinders, one 
inclosed within the other, with 
space between them. The zinc 
is placed in a vertical opening 
in the same side of both carbons, 
through which the fluid can cir- 
culate freely, and is thus brought 
into closer proximity to the con- 
ducting electrode than in the 
Leclanche, and the fluid resist- 
ance thereby reduced and de- 
polarization made more rapid 
and effective. Depolarization is 
also made more effective by the 
increased proportion of surface 
in the conducting electrode to 
that in the zinc. Both electrodes are attached to a close- 
fitting, insulating cover. 

The Diamond Carbon Cell. — The Diamond Carbon Cell, 
shown in Fig. 7, is another of the same class, in which 
the conducting electrode consists of seven round rods 
arranged in a circle around the zinc rod, and put in 
electric connection with each other by attachment to a 
cover made of the alloy known as white metal, which 
is not easily oxidized ; the zinc being insulated by a 
porcelain bushing. 

This cell has the same advantages in regard to de- 




FiG. 6. 



ONE-FLVID CMLLB. 



21 



polarization as the last. The metal cover reduces its 
resistance, and the separate rods are easily renovated 
by heating or soaking in hot water when necessary, and 
cheaply replaced when worn out. 

In other cells of this class, as the Laclede and Mi- 




FiG. 7. 



crophone, the cover is made a part of the conducting 
electrode and the zinc insulated from it. 

The ringing of electric bells is one of the most com- 
mon uses to which sal-ammoniac cells are applied, and 
for which their constancy on open-circuit work especially 
fits them. 

Dry Cells. — A cell constructed with a semi-fluid, not 
liable to spill, is termed dry. Cells filled with sand or 
sawdust, soaked with dilute acid, are instances of this 
construction. Portable cells, having starch or similar 



22 DYNAMIC ELECTRICITY AND MAGNETISM. 

material to absorb the fluid, and hermetically sealed, 
are now becoming common, and are very convenient for 
many purposes ; and, when properly constructed, have 
a high degree of constancy and efficiency. An absolutely 
dry cell is an impossibility ; a certain degree of damp- 
ness or moisture being essential to proper chemical 
action. 

Polarization of One-Fluid Cells. — All one-fluid cells, no 
matter how perfect their construction, are subject to 
polarization to a greater or less degree ; and though 
less complicated than two-fluid cells, and more com- 
venient for many uses, they are not adapted to work 
requiring a continuous current, or in which the intervals 
of rest are not sufficient for complete depolarization. 



I 



2W0-FLUID CELLS. BATTERY FORMATION, 2\ 



CHAPTER IIL 
TWO-FLUID CELLS. BATTERY FORMATION. 

Construction of Two-Eluid Cells. — In the two-fluid cell 
pcJarization is either wholly prevented, or so reduced 
that the cell may be used for work requiring greater 
constancy than can be obtained from a one-fluid cell. 
The construction requires that the conducting electrode 
shall be surrounded with a fluid capable of suppressing 
the hydrogen, while the soluble electrode is surrounded 
with a fluid capable of chemical combination with the 
material of which the electrode is composed; and that 
the means of separation between the fluids shall not be 
such as to prevent electric or chemical action. For this 
purpose a porous cup, like that in the Leclanche cell, 
made of unglazed porcelain, is placed inside the larger 
vessel, and contains one of the electrodes with its fluid, 
while the other electrode with its fluid is placed in the 
outer vessel, and electric and chemical action takes 
place through the pores of this cup, where the fluids 
come into contact. 

Various other means of separating the fluids are used, 
as vessels or partitions of wood, paper, or animal mem- 
brane. Gravitation is also employed; a heavy fluid be- 
ing used in connection with a light fluid, the former 
settling to the bottom of the vessel, while the latter 
rises above it. 

The Daniell Cell. — This is one of the oldest and best 
two-fluid cells in use. In was invented by Daniell, an 
English electrician, in 1836, and has undergone various 
modifications. Fig. 8 represents one of the best known 
styles. The outer vessel is a glass jar containing water 



24 DYNAMIC ELECTRICITY AND MAGNETISM. 



or dilute sulphuric acid, in which is placed a hollow 
cylinder of zinc, having a slit in one side for the free 
circulation of the fluid. Inside this cylinder is placed 
a porous cup containing a solution of copper sulphate 
in water, to which some crystals of the sulphate are 




Fig. 8. 

added; in which is placed a copper cylinder, slit like the 
zinc. 

The chemical reaction is as follows: Hydrogen being 
liberated by the oxidation of the zinc, and the copper 
sulphate (CuSOJ decomposed, the copper (Cu) is de- 
posited on the copper cylinder, and the other constitu- 
ent (SO4) unites with the hydrogen (HJ, forming sul- 
phuric acid (HgSOJ, which in turn is decomposed by 
the zinc (Zn), forming zinc sulphate (ZnSOj, more 
hydrogen being set free to unite with the liberated SO^, 
as before; the interchange taking place through the 
pores of the inner vessel. The hydrogen being thus 



TWO- FLU ID CELLS BATTERY FORMATION. 2$ 



entirely suppressed, depolarization is complete, and the 
copper cylinder, accumulating only pure copper, is al- 
ways in the best condition as an electrode. 

The E. M. F. of this cell is about 1.05 volts. It has 
great constancy, and is but slightly affected by changes 
of temperature ranging from -\-\%° to -|- 100° C.; below 
this range the internal resistance increases, and at —5° 
to —7° C. the solution freezes. Its chief defect is that 
the consumption of material is nearly as great when 
unemployed as when employed. Amalgamation of the 
zinc is not necessary. 

The electric resistance of the porous cup, which re- 
sults from the reduction of the cross-section of the 
fluid in passing through the pores, and from local action 
caused by the material of which this cup is composed, 
led to the invention of cells in which the fluids are sep- 
arated by gravity. 

The Callaud Cell. — One of the best known gravity cells 
is the Callaud, represented by Fig. 9. The copper is 
placed in a solution of copper sulphate at the bottom of 
the vessel, and the zinc suspended in a solution of zinc 
sulphate near the top; the two fluids 
being kept separate by the difference 
in their specific gravity, the copper 
sulphate being the heavier. Connec- 
tion with the copper is made by a 
copper wire, insulated by gutta-percha 
or India-rubber to protect it from 
injury by local action at the junction 
of the fluids, and from contact with 
the zinc. 

The separation of the fluids is never 
quite complete; a certain percentage 
of the copper sulphate rising to the 




Fig. 



upper part of the vessel, producing a copper deposit on 



26 DYNAMIC ELECTRICITY AND MAGNETISM. 

the zinc; an effect which is increased by local action on 
both electrodes, evolving hydrogen and producing 
ascending and descending currents. As this deposit 
accumulates copper pendants are formed, which increase 
in length till they reach the copper sulphate, when 
this action becomes much more rapid, with increased 
waste of the copper sulphate. Hence they should be 
removed before attaining this length, by lightly tap- 
ping the zinc, causing them to drop off. 

This cell has about the same E. M. F. as the Daniell, 
while the reduction of resistance by the removal of the 
porous cup produces a corresponding increase of current 
in the external circuit. 

The Grove Cell. — This cell was invented by Grove, an 
English electrician, in 1839. It is constructed with an 
amalgamated zinc cylinder immersed in dilute sul- 
phuric acid, contained in a glass jar, within which is a 
porous cup containing a strip of platinum immersed in 
strong nitric acid. This acid is rich in oxygen, which 
unites with the hydrogen, producing complete depolar- 
ization; and, being a good electric conductor, greatly 
reduces the resistance. The chemical reaction forms 
water, and also nitric tetroxide (N^O J, which is emitted 
in noxious, red fumes, and is one of the greatest objec- 
tions to this otherwise excellent cell. 

Its E. M. F. is about 1.8 volts, which is 80 per cent 
greater than that of the Daniell cell, while its internal 
resistance is about 20 per cent that of the Daniell. Hence 
a Grove cell of the same size as a Daniell has about nine 
times the current strength. It is therefore one of the 
most powerful cells in use. 

The Bunsen Cell. — In 1843, Bunsen, a German elec- 
trician, adopting the plan originally proposed by Grove, 
produced a cell having carbon instead of platinum in 
the porous cup, but otherwise identical with the Grove 



TWO-FLUID CELLS. BATTERY FORMATIOX. 



cell. This substitution greatly reduced the cost without 
impairing the energy; the E. M. F. and internal resist- 
ance being about the same as in the Grove. 

Depolarization is complete, but the same noxious 
fumes occur as in the Grove cell. 

The Silver Chloride Cell. — Silver chloride was first used 
in the construction of battery cells by Marie Davy 
about i860; subsequently Warren De La Rue made 
such improvements in the construction as to bring the 
cell into general use. His cell, as shown in Fig. 10, 




C- 



( 






2 

Fig. 10. 

consists of a small glass jar, about 5 inches in height 
and i^ inches in diameter, which contains a dilute solu- 
tion of sal-ammoniac, in the proportion of 23 grammes 
to I liter of distilled water, in which is placed a small 
rod of unamalgamated zinc of superior quality; also a 
strip of silver imbedded in a small cylinder of silver 
chloride (AgCl), which is contained in a cylinder of 
parchment-paper. The electrodes are shown separately, 



28 D YNAMIC ELECTRICITY AND MA GNE TISM. 

Z representing the zinc, Ag.Cl. the imbedded silver 
strip, A the paper cylinder, and B the cylinder and 
inclosed strip. The cell is closed by a paraffine stop- 
per fitting air-tight, through which the electrodes pro- 
trude. This prevents evaporation and creeping salts, 
and insulates the electrodes from each other. Near the 
top of the zinc there is a hole into which the silver strip, 
bent over from the adjoining cell, enters as shown at C, 
when the cells are connected into a battery. 

The silver chloride in this cell acts as a depolarizer 
much in the same manner as the copper sulphate in the 
Daniell cell. By its decomposition zinc chloride is 
formed and silver deposited on the conducting electrode; 
hence there is no oxidation, no deposit of hydrogen, 
and consequently no polarization. 

The E. M. F. is 1.03 volts, and the internal resist- 
ance 4.3 ohms. Resistance, being chiefly due to the sil- 
ver chloride, is much greater when the cell is first used 
than subsequently when reduced by the deposit of silver 
throughout the mass of the chloride. 

The small size of this cell makes it convenienjt for the 
construction of batteries having a large number of cells, 
one constructed by De La Rue containing 11,000. 

The construction of battery cells is limited only by 
the number of combinations of suitable materials which 
may be formed; and as the principles which govern these 
combinations have been fully set forth and illustrated 
by the various cells described in the preceding pages, 
it is unnecessary to carry these details farther. 

Battery Formation. — There are two principal methods 
of combining cells to form batteries, known by the terms 
series and parallel. When joined in series, the soluble 
electrode of each cell is connected with the conducting 
electrode of the adjoining cell; and when joined in par- 
allel, all the soluble electrodes are connected with each 



TWO-FLUID CELLS. BATTERY FORMATION. 29 



Other, and likewise all the conducting electrodes. The 
latter method is also known by the terms multiple arc^ 
side by side, and for quatitity; and the series method by 
the term for intensity, to distinguish it from the method 
for quaritity. But as the use of these various terms is 
confusing and unnecessary, it is better to confine our- 
selves to the terms first given above, which have received 
the sanction of leading electricians and are now in gen- 
eral use. 

With a given number of cells a given amount of elec- 
tric energy may be generated, which it is evidently im- 
possible, according to the law of the conservation of 
energy, either to increase or diminish by any method of 
connecting them. 

But it is possible to control and direct this energy in 
such a manner as shall best subserve the uses to which 
it is to be applied; and, for this purpose, either the series 
method or the parallel may be used alone, or the two 
combined to any desired extent. 

Fig. II illustrates the method by which six cells may 
be joined in series; the circles representing cells, and the 
lines conductors. 




Fig II. 
Fig. 12 shows how the same cells may be joined in 
parallel. 




Fig. 12. 



30 DYNAMIC ELECTRICITY AND MAGNETISM. 

Fig. 13 shows a combination of two series of three 
cells each, and these series joined in parallel; and FigJ 




Fig. 13. 

14 shows three series of two cells each, and these three* 
joined in parallel. 




Fig. 14. 

Since electromotive force is that which moves or tends 
to move electricity from one point to another, and de- 
pends on difference of potential, and since this difference, 
in a cell, depends on the nature of its materials and the 
method of construction, w^e should expect to find the 
E. M. F. of a small cell equal to that of a larger one of 
the same composition and construction; and experiment 
proves that such is the fact. 

The case is analogous to that in hydrostatics, where 
liquid pressure depends on difference of level and not 
on the size of the vessel; the liquid in a small vertical 



TWO-FLUID CELLS. BATTERY FORMATION, 3 1 

ube balancing liquid of the same kind contained in a 
larger one connected with it, so that the level is the 
same in each. So when a small cell is joined to a larger 
one of the same kind by connecting the similar elec- 
trodes, so that opposing currents meet, the current from 
the one exactly neutralizes that from the other; proving 
that both currents have the same strength, and hence 
that the E. M. F. of each cell is the same. But this is 
not true of dissimilar cells, differing by construction in 
E. M. F., nor similarly in hydrostatics of liquids differ- 
ing in specific gravity. 

The six cells in Fig. 12, joined in parallel, are practi- 
cally equivalent to one cell six times the size of any 
one of them; for, the similar electrodes of each kind be- 
ing joined together, each set acts as one electrode. 
Hence the E. M. F. of the battery, connected in this way, 
is only equal to that of a single cell; just as in hydro- 
statics the liquid pressure in six tubes of the same size 
placed vertically side by side, and connected with a hor- 
izontal tube at bottom, is only the same as that in any 
one of the tubes alone. But if the six are joined end to 
end in a vertical series, the pressure becomes six times 
as great. So if the six cells are joined in series, as in 
Fig. II, the E. M. F. becomes six times as great. In 
the former instance we have liquid pressure, in the 
latter electric pressure. But since electric resistance va- 
ries directly as the length and inversely as the cross- 
section of a conductor, the resistance also becomes six 
times as great; each of the six cells with its electrodes 
and fluid adding a unit to the length of the conducting 
line, while the cross-section remains the same. And since 
the quantity of electricity passing through a conductor, 
represented by the volume of current, equals the E. M. F. 
divided by the resistance, the quantity obtained from the 
series in Fig. 11 is only one sixth of that obtained from 
the six cells in parallel, as in Fig. 12, and hence no 



32 DYNAMIC ELECTRICITY AND MAGNETISM. 

greater than that of a single cell, though the E. M. F. is 
six times as great. 

When the six cells are joined in parallel, as in Fig. 12, 
the resistance is only one sixth of that developed in a 
single cell; for the cross-section of the united conduct- 
ors is six times as great, affording six times as large an 
avenue for the passage of electricity. Hence, though 
the E. M. F. is only equal to that of a single cell, the 
electric quantity or volume of current is six times as 
great, and hence also six times as great as that of the 
six cells in series, which has been shown to be only equal 
to that of a single cell. 

Hence in the series combination we have current in- 
tensity at the expense of current quantity^ small current 
and large E. M. F.; and in the parallel combination, 
quantity at the expense of intensity, large current and 
small E. M. F.; one being in the inverse ratio of the 
other in each case. 

The combination proper to be used depends on the 
nature of the required work. If there is high resistance 
to be overcome, as in a long telegraph line, the intensity 
must be sufficient to overcome it, and leave a sufficient 
surplus to operate the instruments, and the series 
arrangement should have the preference. But if the 
resistance is low, and the required quantity large, as in 
the deposition of metal in electro-plating, the parallel 
arrangement is to be preferred. 

The practical rule is to make the internal resistance of 
the battery equal to the external resistance to be overcome: 
and our illustrations show that the variation of the rel- 
ative proportions of quantity and intensity by different 
methods of combination is practical for this purpose 
to any required extent. 

The correctness of the rule becomes evident when we 
consider that if by a preponderance of the series arrange- 
ment the internal resistance exceeds the external, 



t:vo-fluw cells, battery formation. 33 

s^reater intensity is developed than is required; and if 
by a preponderance of the parallel arrangement the in- 
ternal resistance is less than the external, the intensity is 
insufficient to overcome the external resistance. In the 
former case there is a waste of intensity at the expense 
of quantity, and in the latter a waste of quantity at the 
expense of intensity. So that the most economical 
arrangement is attained by following the rule given 
above, which is based on the intensity or quantity re- 
quired, to which equality of internal and external re- 
sistance serves merely as a convenient guide. 

In all the various combinations of a given number 
of cells which may be made, as shown, the product ob- 
tained by multiplication of the current strength in 
amperes into the E. M. F. in volts must remain the same, 
since each factor varies inversely as the other; hence 
difference of combination can produce no variation in 
the amount of electric energy developed, as represented 
by this product, the variation observed pertaining ex- 
clusively to the different factors. 

Connection between Cells. — Since all unnecessary re- 
sistance within the battery causes a w^aste of energy, it 
is important that this resistance should be reduced to 
the minimum, both in the cell 
itself, as we have already seen, 
and in the connection between 
the cells. This can often be 
accomplished by clamping the 
electrodes of adjoining cells 
together without any interven- 
ing connection. Fig. 15 shows 
a form of the Grove cell spe- 
cially adapted to this purpose. 
The cell being rectangular, the 
porous cup thin and fiat, and ^^^- ^S- 

the electrodes flat strips, permits a compact arrange- 




34 DYNAMIC ELECrRICITY AND MAGNETISM. 

ment of all the parts; the zinc, Z, being bent so as 
to inclose the porous cup, F, while its upper end is 
clamped in immediate contact with the platinum, P. 

Fig. i6 shows the silver chloride battery, in which the 
ceUs are round but small, permitting them to be placed so 




FiG. 1 6. 

close together that the top of the silver electrode can 
be bent over, and inserted into a hole in the top of the 
zinc. 

Where such methods as the above are not practicable, 
connection can be made by heavy copper wire or strip, 
in which the resistance is insignificant. But it is of the 
utmost importance, in all cases, to have perfect con- 
tacts, kept free from oxidation; and this requires fre- 
quent, careful inspection. 



MAGNETISM. 35 



CHAPTER IV. 
MAGNETISM. 

The Natural Magnet. — The natural magnet is a hara 
black stone, which has the property of attracting iron. 
It derives its name from Magnesia, a country of Asia 
Minor, where it is supposed to have been first dis- 
covered. It was also found at Heraclea, a city of 
ancient Lydia, and hence called also the heraclean 
stone. It was known at least five hundred years before 
the Christian era, being described by Plato and Euripi- 
des. It is very rare, but an ore of iron, closely allied to 
it, known as magnetite, is more abundant, though not 
always magnetic. 

Magnetic Polarity. — No practical use was made of the 
magnet stone till sixteen centuries after its discovery, 
when it was found to have the property of assuming a 
north and south position in the direction of its longer 
axis, when supported so as to have a free horizontal 
movement about its centre of gravity. This property 
was termed polarity, from its reference to the earth's 
poles, and the stone thereafter became known as the 
lodestone — leading stone. It was also observed that iron, 
rubbed with this stone, acquired its properties of attrac- 
tion and polarity, and this led to the invention of the 
mariner's compass. 

The Mariner's Compass. — This instrument at first con- 
sisted of a thin strip of magnetized iron, named from its 
shape the needle, attached to wood or cork and floated 
in a vessel of water; a light wooden pointer, attached to 
it, indicating the ship's course. 



36 DYNAMIC ELECTRICITY AND MAGNEIISM. 

In this rude state it became known in Europe early in 
tne twelfth century, but the exact date and name of the 
inventor are unknown. A much earlier claim for this 
invention is made by the Chinese, but does not seem to 
be well sustained. 

The loss of magnetic energy due to the softness of 
iron seriously impaired the usefulness of the compass, 
but the subsequent discovery of steel furnished the 
material for needles much more permanently magnetic. 
Various improvements followed till it became the per- 
fect instrument which we now have, as represented in 
Fig. 17. The needle is mounted on a pivot, and at- 




FiG. 17. 

tached to the under side of a circular card which ro- 
tates with it, the margin of which is graduated to thirty- 



MAGNETISM, 37 

two divisions indicated by pointers, including the four 
cardinal points N., S., E., W., and also to 360° where great 
accuracy is required. A circular box with glass cover 
incloses it, so poised as to maintain a perfect level un- 
affected by the motion of the ship. The most accurate 
needles are compound, consisting of several needles 
connected together. The compass shown in Fig. 17 has 
eight needles attached to the card, four on each side of 
the axis of rotation. 

The Surveyor's Compass. — This compass differs from 
the mariner's chiefly in having the needle exposed to 
view, the graduated circle stationary, and sights and a 
small telescope mounted above. 

The Earth's Magnetic • Poles.— The polarity of the 
needle was found by Gilbert to depend on the mag- 
netism of the earth, which produces north and south 
magnetic poles by which the needle is attracted; the 
polarity of each being opposite to that of the corresponding 
pole of the needle. These are not identical with the geo- 
graphical poles; the north magnetic pole being near the 
arctic circle, lat. 70° 5' N., long. 96° 46' W., and the south 
near the antarctic circle, about lat. 73° S., long. 154° E., 
according to Airy's maps. Figs. 18 and 19; the location 
of the south magnetic pole being only approximate, its 
position having never been accurately determined. 

There are indications of secondary poles also, but 
their existence and location are not well established; 
neither is it known whether the magnetic poles are 
stationary or slowly changing position, as no accurate 
observations on this point have been made since the dis- 
covery of the north magnetic pole by Ross in June 
1831; previous to which nothing was known in regard 
to the location of either magnetic pole. 

'Declination. — The difference of position between these 
and the geographical poles produces a deflection of t^**. 



38 DYNAMIC ELECTRICITY AND MAGNETISM. 

needle from a true north and south position at all 
points on the earth's surface except those situated on 
what is known as the agonic line, or meridian of no dec- 
lination, and this deflection is termed declination ; the 
needle's north pole being deflected toward the north 
magnetic pole, north of the magnetic equator, and its 
south pole toward the south magnetic pole, south of 
the magnetic equator. The exact declination at any 
point is the angle between the vertical plane of the 
true meridian and that in which the longer horizontal 
axis of the needle lies at the time of observation. 

If the distribution of magnetic force on the earth's 
surface varied unitormly, it is evident that the agonic 
line would coincide with the meridian passing through 
the magnetic and geographical poles, and the declina- 
tion would vary as the distance east or west of this line; 
and the magnetic equator, being equally distant from 
the magnetic poles, would cut the geographical equator 
at opposite east and west points at an angle of about 
20° ; and on this equator there could be no declination, 
horizontal magnetic attraction on each pole of the nee- 
dle being equal and opposite, while the declination on 
any geographical meridian not coinciding with the 
agonic line would vary as the distance north or south 
of this equator, and attain a maximum of 90° at parallel 
points adjacent to either magnetic pole. Hence the 
declination at any point could be calculated from the 
latitude and longitude if the position of the agonic line 
were known. 

But observation shows that this hypothesis is only 
approximately true, and that the distribution of mag- 
netic force on the earth's surface is very irregular, as 
shown by Figs. 18 and 19 ; and that declination, posi- 
tion of the agonic line, and other magnetic facts can be 
determined only by actual observation at each point. 



MAGNETISM. 



39 




Fig. 19. — Southern Hemisphere. 



40 DYNAMIC ELECTRICITY AND MAGNETISM. 

Inclination or Dip. — At the magnetic equator the posi- 
tion of the needle is parallel with the horizon, vertical 
magnetic attraction on each of its poles being equal and 
opposite, but at all points north or south of this line it 
is inclined at an angle known as its dip or inclination ; 
its north pole, north of it, inclining towards the north 
magnetic pole ; and its south pole, south of it, towards 
the south magnetic pole : the inclination attaining a 
maximum of 90° at each, the position of the needle 
becoming vertical. Hence the compass needle requires 
a counterpoise sufficient to counteract the dip and keep 
it in a true horizontal position. 

With uniform variation of magnetic force, the inclina- 
tion at all points between the magnetic equator and 
poles would vary as the magnetic latitude ; but this is 
only approximately true, observation being required to 
determine its value at any point, and also the position 
of the magnetic equator, and parallels, as well as of the 
magnetic poles; and as the south magnetic pole has 
never been definitely located, the vertical position of 
the needle at that point can only be assumed. 

The Dipping Needle. — The inclination is ascertained 
by an instrument known as the dipping needle, constructed. 
with a graduated circle set vertically in the plane of the 
magnetic meridian, around which a delicately poised 
needle has a free vertical movement. 

Magnetic Maps. — By observation of the declination, dip, 
and otherphenomena at various points on the earth's sur- 
face, maps may be prepared which are approximately cor- 
rect for a limited number of years. The maps of Sir 
George Airy, Figs. 18 and 19, and of the U. S. Coast and 
Geodetic Survey, Figs. 20, 21, 22, and 23, have been pre 
pared in this way. The magnetic poles being located 
with approximate accuracy, the magnetic equator is 
found by tracing a great circle connecting all points in 



MAGNETISM, 4 1 

the equatorial region where the needle maintains a per- 
fect horizontal parallel. This circle, which is very 
irregular, cuts the geographical equator at opposite east 
and west points at an angle of about 13°, as shown in 
Figs. 18 and 19. 

The agonic line is found by connecting the points of 
no declination on a great circle passing through the 
magnetic poles and cutting the magnetic equator at 
right angles approximately. Other great circles con- 
necting points of equal declination, and hence called 
isogo7iic lines, pass also through the magnetic poles and 
cut the magnetic equator, in like manner, at approxi- 
mately equal intervals. 

Parallels to the magnetic equator, connecting points of 
equal inclination on the isogonic lines, and hence called 
isoclinic lines ^ cut the agonic line at approximately equal 
intervals. All these lines, both of declination and incli- 
nation, show great irregularities, the irregularities of the 
parallels corresponding approximately to those of the 
magnetic equator. 

Terrestrial Magnetism Illustrated. — If a magnetic nee- 
dle, free to move vertically and horizontally, be brought 
into the vicinity of a magnetized bar of steel, lying in a 
north and south position, be moved directly over it from 
end to end, and also parallel to it at a short distance on , 
each side, it exhibits all the phases of declination and 
dip found on the various parts of the earth's surface, as 
already described, but in a more regular manner; which 
is strong proof that the earth is a great magnet, as al- 
ready stated, with curving lines of force radiating in 
all directions from its magnetic poles, like other mag- 
nets, as described hereafter; thus accounting in a most 
satisfactory manner for the phenomena of declination 
and dip. 

These are the lines represented in part on the maps^ 



42 DYNAMIC ELECTRICITY AND MAGNETISM. 

the needle being merely the instrument by which they 
are traced. 

Magnetic Intensity. — The intensity of this magnetic 
force constantly increases from the magnetic equator 
to each magnetic pole, and is represented at any point 
by the forces producing the declination and dip ; the 
former representing the horizontal component of the 
intensity, and the latter the vertical ; the total intensity 
being ascertained by dividing the horizontal force by 
the cosine of the angle of dip. Hence, representing 
the total intensity by F, the horizontal force by ZT, and 
the angle of dip by ^, we have the usual standard for- 
mula 7^= T.. 

cos u 

There are two methods of ascertaining the relative 
values of the horizontal force at different points, known 
respectively as the methods by oscillation and by deflec- 
tion. 

Magnetic Porce Ascertained by Oscillation. — The oscil- 
lations of the needle, when forcibly deflected from its 
position of rest, are accomplished, like those of the pen- 
dulum under the influence of gravity, in nearly equal 
times, though constantly decreasing in amplitude ; and 
the square of the number of oscillations accomplished 
in a given time, which in the pendulum is proportional 
to the force of gravity, is, in the needle, proportional to 
the horizontal magnetic force. Hence if a represent 
the number accomplished in a given time at any point 
on the earth's surface, and b the number accomplished 
in the same time by the same needle at any other point, 
the relative values of this force at the two points are as 
a^ to b\ 

' Magnetic Force Ascertained by Deflection. — The hori- 
zontal force by which the needle is brought to rest in 
the plane of the magnetic meridian is the resultant of 



MAGNETISM. 



43 



two forces, one tending to rotate it into an east and 
west position, represented by the sine of the angle of 
declination, and the other into a north and south posi- 
tion, represented by the cosine, while the resultant force 
is represented by the hypothenuse of a rigiit-angled 
triangle, of which the sine and cosine form the remain- 
ing sides (see Fig. 45, page 121); the position of the 
needle coinciding with that of the hypothenuse, in which 
the forces are in equilibrium. The relative value of the 
east and west force, by which the needle is deflected, is 
to that of the north and south force, as the ratio of the 
sine to tlie cosine, represented by the tangent. Hence 
the total horizontal force of the earth's magnetism, at 
any point, multiplied by the tangent of the angle of 
declination gives the deflective force at that point. 

Absolute Magnetic Intensity. — The relative magnetic 
intensity being derived, as shown, from division of the 
horizontal force by the cosine of the angle of dip, if the 
absolute value of this force, in C. G. S. units, at any 
point is ascertained, the absolute intensity can also be 
ascertained. To accomplish this two observations are 
necessary with a needle whose magnetic moment or 
force by which it resists deflection is known ; a quantity 
ascertained by multiplying the strength of either pole 
by the length of the needle (or magnet). One of these 
observations, made by oscillation, determines t\\& prod- 
uct of this moment by the horizontal force ; and the 
other, made by the special deflection of a small needle 
by the same needle (or magnet) used in the first obser- 
vation, determines the quotiefit of the moment by the 
horizontal force ; and dividing the product by the quo- 
tient and taking the square root of the result gives the 
absolute horizontal force. 

Previous to 1830 observations on magnetic intensity 
were made by oscillation of the dipping needle, but thi§ 



44 DYNAMIC ELECTRICITY AND MAGNETISM. 

method was found to be inaccurate and the observa- 
tions unreliable. The discovery of the method of ex- 
pressing this intensity in absolute measure was first 
made by Gauss in 1833, and the portable magnetome- 
ter (described in the latter part of this chapter), an im- 
portant aid in such measurement, was constructed by 
Weber in 1836. 

The number of oscillations made in a given time by 
different needles, or magnets, varies as the length, weight, 
form, and polar strength of each, and as the strength of 
the magnetic field in which it is placed. Hence, in 
comparing observations made by different instruments, 
it is necessary to correct any errors which may arise 
from such variation. It is also important to prevent 
errors due to loss of magnetism, by frequent testing, and 
remagnetizing when necessary. 

Parallels to the magnetic equator, connecting points 
of equal magnetic intensity, and hence called isodynamic 
lines, are traced on maps representing either the hori- 
zontal or the total intensity, as shown in Fig.20. 

Biot's Law. — The magnetic intensity at any point on 
the earth's surface varies with the magnetic latitude ; 
to which it is approximately proportional. The mag- 
netic force, emanating from the magnetic poles and ra- 
diating in curves as already stated, not only on the sur- 
face but into the surrounding space in all directions, 
varies inversely as the square of the distance from either 
pole, except as modified in the manner already shown. 
Hence the intensity is greatest at the magnetic poles 
and least at the magnetic equator, and may be ascer- 
tained approximately at any point by Biot's law, which, 
representing the magnetic latitude by /, makes the in- 
tensity proportional to l^i -^ 3 sin^ /. 

Origin of Terrestrial Magnetism. — The origin of terres- 
trial magnetism and its peculiar phenomena is to be 



MAGNETISM. 



45 



found in the reciprocal relations of magnetism and elec- 
tricity as explained in the next chapter, each being ca- 
pable of producing the other. 

Electric terrestrial phenomena have been described 
in the author's " Elements of Static Electricity," Chap- 

ISODYNAMIC MAP OF THE UNITED STATES FOR 1885. 




Cr. S. Coast and t? ^ «^ 

letic Survey. r IG, 20. 

ters XII and XIII, where it has been shown that differ- 
ence of electric potential between different parts of the 
earth's surface and atmosphere is apparentl)^ the result 
of difference of temperature, modified by the unequal 
distribution of land and water; hence the magnetic ter- 
restrial phenomena which we have been considering 
may be regarded as the result of the electric phe- 
nomena; and the peculiar phases of each, as indicated 
by geographical position and otherwise, leave no doubt 



46 DYNAMIC ELECTRICITY AND MAGNETISM. 

of their intimate relationship; so that whether the mag- 
netic phenomena be regarded as a result of the elec- 
tric, or the reverse, both are undoubtedly dependent on 
the same physical influences. 

Secular Variation. — Observation shows that the spe- 
cial phases of terrestrial magnetism are subject to great 
variation in respect to time as well as geographical po- 
sition; such variation being of three kinds, secular, an- 
nual, and diurnal. The first embraces long terms of 
years known as secular periods^ whose length is deter- 
mined by the time in which a complete cycle of changes 
/occurs. The discovery of this variation is due to Gelli- 
brand, an English electrician, and was first published in 
1635. 

The agonic line and the isogonic lines are constantly 
changing position, slowly vibrating between widely 
separated eastern and western limits; hence the declina- 
tion at any point shows a corresponding variation be- 
tween eastern and western maxima; and the time occu- 
pied by the agonic line or any isogonic line in passing 
from its eastern or western limit, on any magnetic par- 
allel, until its return to the same limit again, or by the 
magnetic needle, at any point, in vibrating from its 
eastern or western maximum declination, or elongation, 
until its return to the same declination again, consti- 
tutes a secular period. 

When the declination has attained a maximum, it be-- 
comes apparently stationary, change in the opposite 
direction being for some years imperceptible, after 
which the mean annual variation steadily increases for 
a term of years till the declination becomes zero; a cor- 
responding decrease of annual variation then occurs 
till it again becomes imperceptible, and the declination 
apparently stationary, at the opposite maximum; there 



MAGNETISM. 47 

is then a return through a similar series of variations 
to the original maximum. 

This variation in rate of declination during a secular 
period has its exact analogy in the similar variation of 
rate found in a vibration of the pendulum. 

The length of a secular period is not definitely known, 
as sufficient time has not yet elapsed since observations 
were first made at any point for a complete cycle of 
changes to occur. It varies considerably in different 
parts of the earth; for the United States it is estimated 
at from 250 to 350 years, and for Paris at about 470 
years ; the earliest observations having been made 
there, dating back to 1540. 

Secular vibration does not necessarily imply a change 
in the direction of the needle from west of north to 
east, or the opposite, which occurs only within the 
range of the agonic line; at all points east of that range 
the needle always points west of north, and at all points 
west of it east of north. Neither is it to be understood 
that the vibration of the needle within this range differs 
from that outside of it; the agonic line is simply the 
boundary between east and west declination, and at all 
points within its range the needle changes direction, 
once from east to west of north, and once from west to 
east, during the secular period, according as this line 
vibrates past each point in either direction. 

This would imply that the general vibratory move- 
ment in either direction is simultaneous at all points, 
increase of east declination and decrease of west decli- 
nation, or the opposite, occurring everywhere at the 
same time; and that when either has attained its maxi- 
mum elongation the other has attained its minimum, 
all the lines having swayed to the west or to the east 
simultaneously. But this is never strictly true except 
within a very limited area; declination may have at- 



48 DYNAMIC ELECTRICITY AND MAGNETISM. 



tained its maximum or minimum at a remote point east 
or west of any isogonic line, and tlie opposite phase set 
in long before the same change occurs at intermediate 
points; so that it may be increasing or diminishing in 
opposite directions at the same time on the same par- 
allel. 

Secular Variation in the United States. — It is found that 
this magnetic wave has thus swept across the American 

ISOGONIC MAP OF THE UNITED STATES FOR 1885. 




FTom U. S. Coast and 
Geodetic Survey. 



Fig. 21. 



continent from east to west since observation began, 
and that its return eastward is now setting in. East 
elongation had attained its stationary phase, followed 
by reversal, at Halifax, N. S.,in 1713, at Eastport, Me., in 
1749, at Boston in 1780, at New York in 1799, at Pitts- 
burg in 1808, at Cincinnati in 1815, at Chicago in 1832, 



MAGNETISM. 



49 



at Salt Lake City in 1873, ^^id will attain it at San 
Francisco, as computed, in 1893. 

In 1890 declination, throughout the interior of the 
United States, was tending westward; west declination 
increasing and east declination diminishing, while at 
the extreme eastern and western points it had become 
stationary and the opposite phase was setting in; west 
declination beginning to decrease on the east coast, and 
east declination to increase on the west coast. 




rrom U. S. Coast and 
Gteodetic Sur^'ey. 



Fig. 22. 



The secular periods of dip and magnetic intensity 
are apparently the same as those of declination. 

The earliest reliable observations for the United 
States are those given in Halley's chart for the year 
1700. The agonic line, moving eastward, then passed 



So DYNAMIC ELECTRICITY AND MAGNETISM. 

a little east of Charleston, S. C; decliivalion at that 
point Jan. ist. being 36' E. In 1790 declination had 
attained its maximum eastern elongation, 4° 54yV ^-j 
followed by decrease till about Jan. i, 1890, when the 
agonic line was a little west of Charleston, declination 
being 4tV W. 

Animal and Diurnal Variations. — The annual and diurnal 
variations are apparently due to change of temperature. 
The annual maximum variation occurs in summer and 
the minimum in winter, corresponding respectively to 
the months of greatest and least change of temperature ; 
the diurnal maximum during the day and the minimum 
during the night, corresponding respectively to the 
hours of greatest and least change of temperature. 
These variations are very slight, that of diurnal declina- 
tion being greatest, ranging from 6' to loj', as observed 
at Philadelphia and at London ; while the annual does 
not exceed ij'. 

The electric terrestrial phenomena already referred 
to show annual and diurnal variations corresponding in 
time and amount to the magnetic. 

The Eleven Year Period. — It is found that the greatest 
magnetic diurnal variations take place at regularly re- 
curring periods of about eleven years each, correspond- 
ing to the periods of greatest solar disturbance, as in- 
dicated by the sun-spots, and of most frequent occur- 
rence of the electric phenomena known as the aurora ; 
minimum periods recurring at intervening epochs of 
eleven years. 

Magnetic Storms. — Unusual perturbations in terrestrial 
magnetism often occur, known as magnetic or electric 
storms^ lasting usaulb/ only a few hours, though some- 
times much longer. They are indicated by sudden and 
unexpected deflections of the needle, and great and 
rapid fluctuations from its normal position, and also by 



MAGNETISM, 51 

Other magnetic and electric disturbances, which occur 
simultaneously over extended areas, often embracing 
distant parts of the globe. They usually occur in con- 
nection with the aurora, and are accompanied by electric 
currents in the earth, each phenomenon being doubtless 
due to the same cause, as explained in " Elements of 
Static Electricity," Chapter XV. 

Cosmic Variation. — There is also slight magnetic varia- 
tion due to solar and lunar influence, which may prop- 
erly be termed cosmic. That due to solar influence 
depends on the rotation of the sun on its axis, and 
hence has a corresponding period of about 26 days. 
That due to lunar iufluence exhibits two maxima and 
two minima during each lunar month, the difference 
between which at Philadelphia is about 27", and at 
Toronto about 38". 

Exact Observation. — In view of these numerous varia- 
tions it is evident that the magnetic needle, when light 
and delicately poised, so as to be sensitive to the slight- 
est change, is in a state of constant tremulous motion, 
and never absolutely at rest ; so that the record of an 
observation, to be of true scientific or even practical 
value, must specify the exact time, limited to the day 
and hour when made, as well as the exact location. 
This is especially true of declination, which is of the 
highest practical importance in surveying and naviga- 
tion, often involving important legal controversies. 

Secular Variation at Washington. — Observation at 
Washington began about 1790, at which date the agonic 
line passed through it. In 1797 this line had attained 
its eastern limit, and declination at Washington its 
eastern maximum, being 30' E. Jan. ist. The agonic 
line has since been moving westward, and passed 
through Washington again in 1803. 

Fig 23 shows the position of this line in the United 



52 D YNA MIC ELECTRICI TY AND MA GNE TISM. 



MAP SHOWING THE POSITION OF THE AGONIC LINE IN THE 
UNITED STATES AT FOUR DIFFERENT EPOCHS. 



w. 

7 


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K 




:5^ 


"^ 


^: 


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r 


Si 


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'^Wl 


]7 1 


\ \ 


i 


/ 


v^~ 


-yj 


X 

I 


Mi 


rr~J. 


Lxx 


A^ 


A^ 




^ 




"~r 
1 




V ^k. 


(^ 


C 


■>L 


s 


T 


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N/ 


K7 




J- 


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r> 


■V. if 


^ -Wi 


isliij-Agt 


°^M\(> 


'iV 




/ 




L / 


•/ \ 


P 


d— 








M 


^^ 


V^ 


r^ 




30--nPv 


^^\ 


^-^ 


s \ 




V" 


l> 






^ ^ 


^ 




>\ 


1^ 


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A 


L - — J 


i- 


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\ 

ChJi 


rl^ston 


\^ 




o 


o — 


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J 


Vs. 




o 


o 

30 


-fw-y—- 


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1 



From U. S. Coast and 
Geodetic SurFey. 



Fig. 23. 



MAGNETISM. 53 

States at four different epochs, including that of its 
eastern limit. In 1810 the declination was 12' W.; in 
1830, 39' W.; in 1850, 1° sSylr' W.; in 1870, 2° 55^/ W. ; 
and in 1890, 4° iSyV '^^• 

The mean annual variation in declination from 1790 
to 1810 was 0.6'; from 1810 to 1830, 1.35' ; from 1830 to 
1850, 3.99' ; from 1850 to 1870, 2.85' ; and from 1870 to 
1890, 3.99' ; the average for the entire period from 1790 
to 1890 being 2.55I'. 

Observations on the dip and magnetic intensity in the 
United States date back to the latter part of the last 
century; but, on account of imperfections in the instru- 
ments and methods of observation in general use for 
this purpose previous to 1838, observations made before 
that date are not considered very reliable. The dip at 
Washington was then 71° 13' ; for the next 22 years 
there was alternate increase and decrease, the maximum 
being attained in 1845, when it was 71° 34'. In i860 it 
was 71° 20' ; it then steadily declined, with slight alter- 
nation, at a mean annual rate of about 1.75', and in 1890 
was 70° 24'. 

The total magnetic intensity at Washington in 1840 
was 0.61923 of a dyne. It decreased from that da'ie to 
1850, when it was 0.61370 ; increased to 0.61877 i^^ 1865, 
and decreased to 0.60863 in 1885. 

Secular Variation at San Francisco. — The declination at 
San Francisco in 1790 was 13° 6' E.; in 1810, 14° 6' E.; 
in 1830, 15° E.; in 1850, 15° 47yV E.; in 1870, 16° 20^' E.; 
and in 1890, 16° 34x^0' E. 

The mean annual variation in declination from 1790 
to 1810 was 3' ; from t8io to 1830, 2.7' ; from 1830 to 
1850, 2.37' ; from 1850 to 1870, 1.65' ; and from 1870 to 
1890, 0.72' ; the average for the entire period from 1790 
to 1890 being 2.088'. 



54 DYNAMIC ELECTRICITY AND MAGNETISM. 

There are no accurate data from which variation in 
dip and total magnetic intensity on the west coast of the 
United States, for an extended period, can be ascer- 
tained, reliable observation on these phenomena being 
very recent. The dip at San Francisco in 1885 was 
62^ 15', was thought to have just passed its maximum 
and to be slowly decreasing ; and the total magnetic 
intensity for the same date was about 0.5456. 

Artificial Magnets. — Various metals besides iron and 
steel can acquire magnetism, but only in a slight degree, 
especially nickel and cobalt, also chromium, cerium, and 
manganese ; and Faraday found that all substances 
apparently are susceptible of magnetic influence, as 
might be inferred from the magnetism of the earth 
itself. But steel of high temper is the only metal ca- 
pable of both acquiring and retaining magnetism to a 
sufficient degree for practical purposes, hence all perma- 
nent artificial magnets are made of it ; and magnetized 
steel, like the natural magnet, is capable of magnetizing 
steel or iron brought into contact with it without im- 
pairing its own magnetism. 

Steel magnets may be of any convenient size and 
form, but are usually made either straight or U-shaped, 
2 to 12 inches in length, \ inch to an inch or more in 
width, and ^-^ to f of an inch in thickness. They may 
be magnetized to a certain degree by simple contaj_ 
with a natural or artificial magnet, also by electricity 
or, more effectually, by the electro-magnet, as explained 
hereafter ; or, if straight, by placing them for a con- 
siderable time parallel to the line of inclination in the 
plane of the magnetic meridian. 

This process may be hastened by concussion or, in 
the case of a wire, by torsion, both indicating that mag- 
netism is a molecular effect, the molecules being under a 



MAGNETISM. 55 

Strain in this position to which they yield more rapidly 
when assisted by the concussion or torsion. 

A common method is represented by Fig. 24. Two 
magnets having their opposite poles in contact and 




.NS. 



]N 



Fig. 24. 

resting on the bar to be magnetized, at its centre, are 
drawn apart to its opposite ends and brought together 
again at the centre repeatedly an equal number of times 
on its opposite surfaces till it becomes magnetically 
saturated, the final movement on each surface terminat- 
ing at the centre. The polarity acquired by the bar at 
each end is opposite to that of the magnetic pole brought 
into contact with it. Bars of the U or horseshoe form 
are magnetized in a similar manner by taking the centre 
of the bend as the point for beginning and terminating. 

Another method is to interpose a non-magnetic body 
between the magnetizing poles and move both alternately 
from end to end over each surface an equal number of 
times. 

Magnetic Saturat?ion. — By magnetic saturatto?i is meant 
the full quantity of magnetism which the bar is capable 
of retaining. It may be magnetized above the point 
of saturation, when it is said to be stcper-saturated, but 
the extra magnetism thus acquired is rapidly dissipated. 

The Armature. — The magnetism of straight bar mag- 
nets and magnetic needles becomes impaired in the 
course of years, and the needles especially require to be 
remagnetized to maintain the requisite strength. The 
U and horseshoe magnets are each furnished with a 
piece of soft iron connecting the poles, and held there 



5 6 DYNAMIC ELECTRICITY AND MAGNETISM, 

by magnetic attraction. It is known as the armature or 
keeper^ from its supposed ability to prevent magnetic 
loss by completing the magnetic circuit. The chief ad- 
vantage of the U or horseshoe form is in the concent 
tration of the magnetic attraction of both poles on the 
same armature, which is itself thus temporarily mag- 
netized and has poles of opposite magnetism to those 
by which it is attracted, which increases the force of 
attraction. 

Two bar magnets placed near each other, side by 
side, can also have armatures with advantages similar 
to those just mentioned. The term armature is also 
similarly applied, in the construction of electro-magnetic 
apparatus, to a piece of iron attracted by a single pole; 
also in a special, technical sense, in the construction of 
the dynamo, as explained hereafter. 

Laminated Magnets. — Magnets of either form men- 
tioned are often made of a number of thin bars, sepa- 
rately magnetized and bound together with their similar 
poles in contact. They are known as laminated, and 
have greater magnetic strength than those having the 
same amount of steel in a single piece, an effect proba- 
bly due to the partial suppression of magnetic eddy 
currents, to which the steel massed in a single thick 
piece is liable. Such currents occurring within the 
mass, like electric " local action" within the battery cell, 
tend to neutralize and reduce magnetic potential differ- 
ence, while their external effect is lost. But the lami-j 
nated structure tends to confine the lines of force to the' 
laminoe and give them a normal direction toward the 
poles. These eddy currents were first observed by 
Foucault, and hence are termed Foucault currents. In 
large electro-magnets special construction is required 
to guard against their deleterious effects. 

Magnetic Loss. — In addition to the loss by gradual 



MAGNETISM. S7 

dissipation, already referred to, steel magnets lose their 
magnetism by sudden or violent perturbations, such as 
concussion, a white heat, or such extreme cold as — ioo° 
C, which indicates, as before, that magnetic change is 
dependent on molecular change. 

Magnetized iron rapidly loses its magnetic energy, 
especially iron of the softer grades, but retains a small 
amount known as residual magnetism ; a similar amount 
being often acquired in the process of the manufacture 
of iron machinery. 

Portative Force. — The attractive property of the mag- 
net is manifested in sustaining pieces of iron or steel 
suspended from it, both by direct contact and through 
the medium of other pieces, so that a number of small 
pieces, as nails or needles, may be suspended from each 
other. This property is known as its portative force. If 
connection with the magnet be severed, the iron quickly 
loses its magnetism, but tne sceel retains it to an extent 
governed by its mass and temper. 

Magnets of the U or horseshoe form can sustain 
weights attached to their armatures to the amount, in 
some cases, of twenty times their own weight, and a 
little lodestone mounted in Sir Isaac Newton's signet- 
ring could sustain two hundred times its own weight. 

The portative force does not vary in the direct ratio 
of the mass; the proportion of force to mass being 
greater in small than in large magnets. The following 
rule is given by Bernoulli: 

Let/ represent the force, ^e/ the weight of the magnet, 
and a the quality of the steel and method of magnetiz- 
ing; then/ = rtr Y 2e/. 

The weight sustained varies also somewhat as the 
area of surface contact between the magnet and its 
armature, and the portative force is gradually increased 
by frequent additions to the load, but this increase is 



58 DYNAMIC ELECTRICITY AND MAGNETISM. 

lost by sudden separation of the armature from the 
magnet. 

Polar Attraction and Repulsion. — If the similar poles 
of two magnetic needles or magnets, free to move, are 
placed in mutual proximity they repel each other, but 
if their dissimilar poles are so placed they attract each 
other; from which is derived the law that like magnetic 
poles repel and unlike attract each other. 

From this it will be seen that the poles of the needle 
are opposite in polarity to those of the earth by which 
they are attracted, but for convenience the pole which 
points north is termed the north pole, and that which 
points south the south pole; though they are sometimes 
more appropriately termed the north-seeking and south- 
seeking poles. Their initial letters, ^and -S, are used to 
distinguish them, magnets being usually marked on the 
north pole only, by i\^ or a cross-line; hence the expres- 
sion " marked pole'* is sometimes used to distinguish it 
from the south or "unmarked pole." 

It is impossible to produce one kind of polarity with- 
out at the same time producing its opposite. This 
becomes manifest in a very striking manner when a 
magnet is broken, the pieces assuming opposite polarity 
on opposite sides of the fracture and at opposite ends, 
each becoming a perfect magnet. If the parts be pressed 
closely together in their original posnion these poles 
disappear, leaving only the poles at the original ends; 
and the same thing occurs if the opposite poles of two 
rectangular magnets of the same cross-section be pressed 
together so as to make perfect joint. 

A magnetic pole may have sufficient strength to over- 
come by induction the repulsion of a weaker one of 
similar polarity and produce attraction when brought 
sufficiently close, while at a greater distance where the 
induction is less there is repulsion. 



MAGNETISM. 59 

Unmagnetized iron or steel is attracted by either pole 
of the magnet, and apparently attracts either pole, but 
only as the result of reaction, being itself magnetically 
passive. 

Magnetic Lines of Force. — If a sheet of paper be placed 
over a bar magnet and iron filings dusted over it, the 
sheet being lightly tapped, the filings arrange them- 
selves in curves corresponding to the lines of force em- 
anating from the poles, as shown in Fig. 25. Each 
filing becomes itself a magnet by induction, and their 




Fig. 25. 
dissimilar poles being, mutually attracted, attach them- 
selves to each other, while their similar poles are 
mutually repelled; hence poles of the same name all 
point in the same direction. 

The lines of force, radiating from each pole of the 
magnet, being mutually repelled and attracted toward 
the opposite pole, are under the influence of two forces, 
each varying inversely as the other, the one urging them 
directly forward and decreasing as the square of the 
distance from each pole increases, the other attracting 



6o DYNAMIC ELECTRICITY AND MAGNETISM. 

them toward the opposite pole, and increasing in the 
same ratio as the first decreases, becoming greater as 
the distance to the opposite pole lessens and the dis- 
tance from the originating pole increases ; hence the 
resulting curves are formed as indicated by the filings. 

The space inclosed by lines of force is termed a tube 
of force. 

Magnetic Field. — The filings show the lines of force 
only in the plane of the sheet of paper, which may be 
placed at any angle at which they can be sustained in 
position ; while close to the poles they stand on end 
nearly at right angles to the paper. Hence it becomes 
evident that the lines of force inclose the magnet in the 
form of a spheroid which is cut by the plane of the 
paper and shown in longitudinal section. The friction 
of the filings on the surface, and their weight, inertia, 
and tendency to mass together, prevent free movement, 




Fig. 26. 

so that they indicate the actual position of the lines of 
force very roughly. Those lines must be understood to 
fill the entire space inclosed by the spheroid, constitut- 
ing what is known as the magnetic field ; a more accurate 
conception of which would be such a figure as might be 
supposed to form itself around a magnet suspended in 
an atmosphere of iron vapor. 

In Fig. 26 the filings represent what are known as 



MAGNETISM, 



6i 



consequent poles, which may result from imperfections in 
the temper of the steel or in the method of magnetiz- 
ing. Such poles may also be produced in a thin bar of 
highly tempered steel by touching it at several points 
with a magnet. The result in either case is similar to 
that of several short magnets joined together by their 
opposite poles. 

In Fig. 27, A and B, the filings show repulsion and 





Fig. 27. 

attraction in a very instructive manner. A represents 
like poles in proximity with the lines curving away 
from each other, while B represents unlike poles in a 
similar position with the lines curving towards each 
other. 

In Fig. 28 is shown at A how the opposing lines of 
force from like poles produce mutual repulsion as al- 
ready described, while at B the curving lines from un- 
like poles interlock and produce mutual attraction. 

Since these lines do not radiate into vacant space, but 
into the air, it may safely be assumed that the air is the 
medium by which the magnetic field is formed, and 
that it becomes magnetized in a manner similar to that 
of the iron filings by which the field is represented in 
section ; which accounts rationally for the observed at- 
traction and repulsion. This hypothesis is strengthened 
by the consideration that it is impossible for energy, in 
any form, to exist independent of matter, so that the 
field could not be formed in an absolute vacuum • mag- 



62 DYNAMIC ELECTRICITY AND MAGNETISM. 

netism itself being doubtless an effect of energy acting 
on matter. Hence we must either assume that the mat- 
ter, in this case, is the air of which we have actual 




knowledge, or the hypothetical ether whose actual ex- 
istence has never been demonstrated. 

The interposition of any substance in the magnetic 
field except that of magnetic bodies, as iron or steel, 
or those termed diamagnetic, as bismuth and copper, 
offers no obstruction to magnetic induction ; attraction 
or repulsion taking place through all other bodies with 
the same facility as if they were not present : while 
steel or iron, especially soft iron, absorbs and diverts 
the lines of force through its own substance in propor- 
tion to its mass, extent, and relative position. Iron may 
therefore be used as a shield against magnetic influ- 
ence. 

Diamagnetic bodies also offer slight obstruction, but 



MAGNETISM. 63 

in an opposite sense to that of iron or steel, resisting or 
turning aside lines of force instead of absorbing them. 

Form of Magnets. — The forms of the magnet already- 
described are the most convenient for practical use ; but 
iron or steel in any form may be magnetized, becoming 
polar normally in the direction of its longer axis, which 
would be true of magnets having the form of the spher- 
oid, cylinder, or ellipse, as well as of the more common 
forms ; but in such forms as the sphere or circle, having 
equal radii, the separate poles are not distinguishable, 
but must be supposed to exist and neutralize each 
other. 

Opposite surfaces of sheet iron or steel may be so 
magnetized as to acquire opposite polarity ; such mag- 
netic distribution being known as lamellar^ in distinction 
from the ordinary distribution, which is termed solenoidal^ 
and such sheet magnets are known as magnetic shells. 

Magnetic Penetration. — The depth to which magnetism 
penetrates depends somewhat on the degree of magnet- 
ization and the size of the bar in cross-section. It is 
strongest in the outer layers, as may be shown by re- 
moving them with sulphuric acid, when the magnetism 
will be found to become constantly weaker as the cen- 
tral core is approached. The same may also be shown 
by magnetizing bundles of thin steel plates bound to- 
gether and gradually removing the outer ones ; also by 
means of a magnetized steel tube, whose magnetism 
is found to be nearly equal to that of a solid bar of the 
same cross-section. 

Location of the Poles. — The poles have been thus far 
assumed to be at the ends of the magnet's longer axis, 
but this is practically true only of long, thin, narrow 
magnets uniformly magnetized throughout ; their true 
location is a little inside of the ends, the distance vary- 



64 DYNAMIC ELECTRICITY AND MAGNETISM. 

ing directly as the mass in cross-section, so that in 
thick magnets it becomes noticeable. 

Paramagnetic and Diamagnetic Bodies. — Faraday pro- 
posed to call such bodies as are capable of assuming the 
magnetic properties of attraction and repulsion, as iron, 
steel, nickel, and cohsXt, par a7nagnetic, while bodies which 
are repelled by either magnetic pole, as bismuth, anti- 
mony, copper, and phosphorus, he proposed to call dia- 
magnetic. But these terms have been adopted only in 
part, the utility of " paramagnetic" especially being 
questioned. 

Some writers accept the term diamagnetic as applied 
to the latter class of bodies, and designate the former 
as magnetic, a usage which is simpler and more con- 
venient. 

Magneto-Crystallic Induction. — Bodies, whether trans- 
parent or opaque, having a crystalline structure are in- 
fluenced by magnetic action differently from bodies 
which do not possess such structure, and diamagnetic 
c-rystalline bodies differently from magnetic crystalline 
bodies. Such a body when suspended subject to mag- 
netic induction is most strongly influenced in a certain 
direction known as its magne-crystallic axis, which, in 
crystals having cleavage, is usually at right angles to 
the plane of cleavage. This axis, according to Tyndall, 
seems to lie in the direction of the crystal's greatest 
density, and magnetic crystals, free to move, take posi- 
tion with this axis in the direction of the lines of force, 
while in the position assumed by diamagnetic crystals 
it is at right angles to those lines. 

The whole subject is imperfectly understood, and 
opinions in regard to it are conflicting ; and In its pres- 
ent aspect it must be regarded as of secondary impor- 
tance. 



Magnetism, 65 

Magnetism as a Mode of Molecular Motion. — The mag- 
netic phenomena thus far observed indicate that mag- 
netism is closely related to the molecular constitution 
of the magnetized body. The effects of extreme heat 
or extreme cold, of concussion and torsion in producing 
or destroying magnetism, all of which affect the molecu- 
lar constitution, have already been noticed; it is also 
found that when iron filings are closely packed in a 
tube and magnetized the mass exhibits all the magnetic 
properties of a bar magnet, but if disturbed by being 
shaken up the magnetism disappears. The filings in 
this case may roughly represent the molecules of a solid 
bar, and the magnetic loss is analogous to that produced 
in such a bar by concussion. If the tube is of glass the 
filings can be seen to arrange themselves lengthwise, 
with similar poles all turned in the same direction, as 
already observed in the curved lines of filings, con- 
stituting magnetic series, from which results the polarity 
of the mass, as in magnets with consequent poles. 

It is found that magnetizing a steel bar produces a 
slight change in its form, the bar becoming a little 
longer, and its other dimensions being correspondingly 
reduced; an effect attributed to a change of position in 
its molecules. In the unmagnetized bar the molecules 
may be supposed to be massed together without order, 
but under the magnetic influence to arrange themselves 
symmetrically in the direction of their longer axes, with 
similar poles in the same direction, like the filings, the 
results being the change of form mentioned above and 
the polarity of the bar. This theory receives further 
support from the slowness with which steel acquires 
magnetism, as compared with soft iron, and its power of 
retention, while the iron both acquires and loses it 
rapidly; this quality in the steel being attributable to its 
rigidity, which resists change of position in the mole- 



66 DYNAMIC ELECTRICITY AND MAGNETISM. 

cules, while those of soft iron easily yield to such change 
Further proof of the same character is found in the fact 
that when a bar is magnetized suddenly by electric 
action a clink may be heard in it, both at the beginning 
and end of the process, which can be satisfactorily ac- 
counted for only on the theory of molecular action. 

The above phenomena clearly indicate molecular 
change of position as a result of magnetization, and 
hence motion; but if the motion should cease when the 
molecules have assumed symmetrical position, magnetic 
action should also cease, for it would be absurd to sup- 
pose that this action could continue when the motion 
which gave rise to it ceased, and that it should be the 
result of mere symmetrical arrangement, the molecules 
thereafter remaining quiescent: but we find, on the con- 
trary, that it is then at its maximum, and, in steel, re- 
mains permanent for years. Hence we must infer a 
corresponding maximum and continuity of molecular 
motion. 

The character of this motion cannot be known. We 
may suppose each molecule to oscillate in the direction 
of either its longer or shorter axis, or to rotate around 
either axis, or to have a vortical motion, or to combine 
two or more such motions; but it is a reasonable infer- 
ence that this motion, whatever its character, is similar 
and uniform in each molecule, so that there is no inter- 
ference between them such as would result from a dif- 
ffirence either in their motions or position. And this 
condition may be supposed to constitute the difference 
between the magnetized and unmagnetized bar; or 
more explicitly, that this motion is itself that which we 
term magnetism. 

Hence if we could be endowed with some superior 
sense, capable of penetrating the magnet and revealing 
its separate molecules and their motions, we should 



MA GNE TISM. 6^ 

probably see, instead of a quiescent body, a quivering 
mass of innumerable atoms, each moving with incon- 
ceivable rapidity, and all in a uniform manner and in 
obedience to a common impulse. 

This theory of magnetism has the sanction of Clerk 
Maxwell, Hughes, and others, and is in accordance with 
the similar theories in regard to heat and electricity 
among whose advocates Tyndall is prominent. And 
as we have seen that different kinds of motion may 
be combined in the same molecule, without interfer- 
ence, as in larger masses, we may attribute the heat to 
one kind, the electricity to another, and the magnetism 
to a third, all being different manifestations of that 
universal energy which is inherent in all matter. 

The theory of magnetism being a fluid, or two dis- 
similar fluids, once so popular, has now become obsolete, 
and cannot be sustained en rational grounds in the light 
of recent investigation. 

Analogy between Magnetic and Electric Phenomena. — 
The close analogy between many of the phenomena 
of electricity and magnetism indicate that both are 
closely allied, if not identical, as will appear more fully 
in the next chapter. But it may here be noticed that 
opposite magnetic polarity has its analogy in opposite 
electric polarity; that in both cases the opposite kinds 
are coexistent and neutralize each other; that magnetic 
distribution is influenced by the form of bodies in a 
manner similar to that of electro-static distribution; and 
that magnetic attraction and repulsion has its analogy 
in electric attraction and repulsion. But in all these 
analogies there is a well-defined difference observable 
which easily distinguishes the magnetic from the electric 
phenomena. 

Coulomb's Torsion Balance. — A full description of this 
instrument and its application to the measurement of 



6s D YNA MIC RLE CTRtCtT V A ND MA GNE TISM, 




electric force may be found in the writer's *' Elements of 

f Static Electricity." It is used in 
a similar manner to measure 
magnetic force, as shown in Fig. 
29. It consists of a circular glass 
case, with a vertical cylinder 

projecting from the cover, hav- 
ing at its upper end a graduated 
circle, with a pointer to move 
round it attached to a milled 
head, from which is suspended, 
by a fine wire, a magnetic 
needle, with its poles opposite a 
circle on the case, graduated to 
correspond to the one above. 
The following experiment 
Fig. 29. made by Coulomb shows its 

use: Zero of the lower scale being brought opposite 
one of the needle's poles, and accurately adjusted in 
this position by comparison with a copper needle of 
equal weight suspended by a thread, and the upper 
scale being adjusted with its zero opposite the pointer, 
it was ascertained, by a preliminary trial, that the 
milled head required to be turned 35° to produce 1° of 
deflection in the needle; hence magnetic force, in this 
case, was to torsion as i to 35. A magnet was then in- 
serted, as shown, with its pole close to the similar pole 
of the needle, and was found to produce 24° deflection; 
hence its force, as ascertained above, was 35 times 24°, 
to which must be added the 24° of torsion, giving 
24° X 35 + 24° = 864° as the " torsion equivalent " of mag- 
netic repulsion with the poles 24° apart. 

The milled head was then turned so as to reduce this 
distance one half (12), requiring 8 complete turns, 
which gives 8 X 360" = 2880°. But the 12° remaining 



MAGNETISM. 69 

at bottom represented a force equal to half the original 
" torsion equivalent," or 432°, which must be added in, 
giving 2880° + 432° = ZZ^'^'^i nearly four times 864°, as the 
" torsion equivalent" of magnetic repulsion at one half 
the distance. 

In like manner it can be shown that any reduction of 
distance, algebraically represented by ^, would require 
a '' torsion equivalent" equal to c^\ hence magnetic force 
is thus proved to vary inversely as the square of the dis- 
tance. 

The inaccuracy observable in the arithmetical result 
is accounted for by inaccuracies in the instrument, and 
in the angular measurement adopted, which are fully 
explained in " Elements of Static Electricity" referred 
to above. 

The Gauss-Weber Portable Magnetometer. — This instru- 
ment, used for measuring the horizontal force of the 
earth's magnetism at any point, as shown by the mag- 
netic declination, is constructed as follows: A bar mag- 
net of convenient size is suspended horizontally, from a 
vertical standard, by an unspun sillc fibre. To one end 
of this magnet is attached a lens, and to the other a 
glass scale adjusted to the lens's focus of parallel rays. 
This part of the apparatus is inclosed in a box having 
a small window at each end, on a line with the hori- 
zontal axis of the magnet, through one of which light is 
admitted to the rear of the scale, and through the op- 
posite one the parallel rays from the lens pass out and 
enter the field-glass of a small telescope, mounted in 
front, through which the scale divisions may be ob- 
served. 

This apparatus is mounted on a tripod on which it 
can be rotated horizontally around the axial line of sus- 
pension of the magnet, and the angle of rotation meas- 



70 DYNAMIC ELECTRICITY AND MAGNETISM. 

ured on an azimuth scale with vernier attachment, 
mounted on the tripod underneath the apparatus. 

The instrument being adjusted to the proper level, 
the torsion is first removed from the silk fibre by sus 
pending from it a small plummet of the same weight as 
the magnet, after which the magnet is suspended so as 
to come to rest in the magnetic meridian without pro- 
ducing torsion of the fibre, and the movable part of the 
apparatus rotated till some division of the magnet scale 
coincides with the cross-wires in the field of the tele- 
scope. The reading on the lower scale is then noted, 
and by a second rotation the instrument is adjusted to 
the true north, ascertained by observation of one of the 
heavenly bodies by means of a small transit apparatus 
mounted back of the magnet-box ; and the reading of 
the low^er scale for this second position being noted and 
the necessary corrections made, the difference of the 
two readings gives the true declination for the place 
and time. 



;» 



ELECT R OMA GNE TISM, 7 1 



CHAPTER V. 
ELECTROMAGNETISM. 

Deflection by the Electric Current. — Such accidental 
effects as the magnetizing of steel instruments by light- 
ning had long indicated some relation between mag- 
netism and electricity, but all attempts to produce 
similar results by artificial means had failed to give 
satisfactory results. In 1802 ' Romagnosi of Trente 
noticed that the magnetic needle was deflected by the 
voltaic current, but his discovery failed to attract at- 
tention. In 1819 Oersted of Copenhagen discovered 
that the needle was not only deflected by the voltaic 
current, but tended to take a position at right angles to 
it, and that the deflection was governed by the direc- 
tion of the current and relative position of the needle. 
The discovery, like that of Volta, marks an important 
epoch in electric progress; it established beyond doubt 
the mutual relationship of electricity and magnetism, and 
was the origin of the science of electromagnetism with 
all the great inventions to which it has given rise. 

Oersted's experiments are easily repeated by holding 
a straightened section of copper wire, connecting the 
poles of a battery or cell, alternately above and below 
a poised magnetic needle, and reversing the direction of 
the current in each position. 

The Galvanoscope. — A better instrument for this pur- 
pose is represented by Fig. 30, consisting of a mounted, 
rectangular brass frame, surrounding a poised needle 
lengthwise, and provided with binding-screws at the 
terminals of the wires. The under wire has its ter- 



72 DYNAMIC ELECTRICITY AND MAGNETISM. 



^ 




minals at A and B^ and the upper wire, which is joined 
B c to the under at the left and in- 
^ sulated from it at the point of 
support on the right, has its ter- 
minal at C. Hence when the 
battery wires are attached to A 
and B the current flows under 
the needle; when attached to A 
and C, over the needle; and when 
attached to B and C, round the 
needle; its direction below being 
Fig. 30. ^j^g reverse of that above. 

When the frame is parallel to the needle in the mag- 
netic meridian and the current is flowing over the 
needle from north to south, the north pole is deflected 
to the east; when the current is reversed the deflection 
is to the west; and when the current flows under the 
needle from north to south or from south to north these 
deflections are reversed. 

When the flow is round the needle lengthwise in 
either direction, through the connections at B and C, 
the deflecting force is doubled, the current in the upper 
and under wires flowing in opposite directions, and 
hence both tending to deflect in the same direction, as 
shown above. 

Since this instrument may be used to indicate the 
presence and direction of an electric current, it is known 
as the galvanoscGpe. 

The Schweigger Multiplier. — By multiplying the coils 
which pass round the needle the deflecting force may be 
proportionately increased within certain limits. This 
may be done by winding the wire, provided with an in- 
sulating envelope, on a frame of non-magnetic material. 
The effect may be increased by using a short needle 
which shall be included within the helix at any angle of 



ELECTRO MA GNE TISM. 73 

deflection, and which, for convenience of observation, 
may be connected with a light non-magnetic pointer. 

The first instrument of such construction was called 
the Schiveigger multiplier^ in honor of its inventor. 

Ampfere's Rule. — To determine the direction of the 
deflection in every case Ampere proposed the con- 
ception of a little human figure so placed that the cur- 
rent would enter at its feet and leave at its head, its 
face being turned constantly towards the needle and its 
arms extended at right angles to its sides. The left 
hand would then constantly indicate the direction of 
the north pole's deflection, and the right that of tlie 
south pole, at any point, above, below, or at either side; 
the deflection in the latter case tending vertically. 

The Astatic Needle. — The force which deflects the 
needle as above acts at right angles to that of the 
earth's magnetism which tends to maintain it in the 
plane of the magnetic meridian, and the amount of de- 
flection depends on the relative strength of the electric 
current, the force of the earth's magnetism at any point 
being practically constant; but the deflection evidently 
can never, under these conditions, equal a right angle. 
But if the effect of the earth's magnetism is neutralized 
in the apparatus, a much more sensi- 
tive and effective instrument can be 
produced. 

This is done approximately by the 
astatic needle, shown in Fig. 31, which 
consists of two parallel needles of equal 
length attached to a short vertical 
support, with their poles reversed, so 
that each neutralizes the directive ^^^- 3i' 

force of the other. They are usually suspended by 
an untwisted silk fibre so as to be uninfluenced by 
friction, protected from air currents by a glass case, and 




74 DYNAMIC ELECTRICITY AND MAGNETISM. 

provided with a graduated scale to indicate tlie amount 
of deflection. 

The wire carrying the current passes round one of 
the needles lengthwise, or may pass round each, being 
wound alternately in opposite directions; and the poles 
being also reversed, it is evident that the current flow- 
ing in each section of the wire must produce deflection 
in the same direction in both needles. 

But it is practically impossible to construct the two 
needles with such mathematical precision that there 
shall not remain a slight deviation in mass, magnetiza- 
tion, and parallelism sufficient to produce a prepon- 
derance influenced by the earth's magnetism; so that 
the very best astatic needles are only approximately 
correct. 

Compensating Magnet. — A similar astatic effect is pro- 
duced by fixing a magnet in the magnetic meridian 
wdth its poles above the similar poles of the needle, and 
at such a distance that its influence is just sufficient to 
counteract the directive force of the earth's magnetism. 
If too close it reverses the needle's position, but with a 
provision for vertical adjustment it can be maintained 
in the best position for directive compensation. 

Cause of Deflection. — If a copper wire in w^hich an elec- 
tric current is flowing pass at right angles through a 
card on which iron filings are dusted, the filings, when 
the card is lightly tapped, arrange themselves in con- 
centric circles around the wire, indicating that lines of 
force, due to the current, circulate in planes at right 
angles to it and magnetize the filings. 

If we suppose such a wire to pass through this page, 
at right angles to the paper, the current to flow from 
the reader, and a magnetic needle to be carried round 
it, the needle would constantly tend to assume position 
in a plane parallel to the paper, its north pole, by Am- 



ELECTROMAGNETISM. 75 

pere's rule, turning in the same direction as watch- 
hands move; while if the current flowed toward the 
reader this direction would be reversed. 

From such indications it is evident that there is 
around every current-bearing wire an electric field in 
which lines of force circulate in planes at right angles 
to Its length; and since it has already been shown that 
lines of force in the magnetic field circulate in planes 
parallel to the magnet's length, it is evident that the 
tendency of these forces, when brought into mutual 
proximity, must be to cause the needle or magnet to 
take position at right angles to the direction of the cur- 
rent, in which position the planes of the magnetic and 
electric forces coincide. 

We may assume a similar physical condition for the 
electric field to that already assumed for the magnetic, 
namely, that the air, or the hypothetical ether, is the 
medium by which the force radiates, and is itself elec- 
trified throughout a space of which the current-bearing 
wire is the central core. 

The Electromagnet. — It was discovered in 1820 by both 
Arago and Davy that iron and steel could be magnet- 
ized by the electric current by inclosing a bar of either 
metal in a helix of insulated wire through which a cur- 
rent is passing; the steel remaining permanently mag- 
netic after being withdrawn from the helix, while the 
iron is magnetic only while inclosed and during the pas- 
sage of the current ; hence the latter is technically 
known as the electromagnet^ in distinction from the 
former. 

Electromagnetic Poles. — By observing the direction in 
which the current from the battery or other electric 
generator is passing, the poles may easily be distin- 
guished by Ampere's rule, already given ; that being 
the south pole, viewed endways, around which the cur- 



'j6 DYNAMIC ELECTRICITY AND MAGNETISM. 

rent is passing in the same direction as watch-hands 
move, while that is the north pole around which it 
flows in the opposite direction. 

Winding. — It is immaterial in which direction the 
helix is wound, whether from right to left, or from left 
to right, or in layers in either direction, alternately 
from end to end, like thread on a spool, provided the 
winding, in each case, is in the same direction through- 
out; but if reversed, in sections or in alternate layers, 
the result is consequent poles at the points of sectional 
reversal, or neutralization between layers oppositely 
wound. 

Magnetic Strength. — An electromagnet is capable of 
acquiring magnetic strength, as represented by the force 
in any way exerted, far in excess of the best steel mag- 
net of similar size, and they have been made of suffi- 
cient lifting power to sustain more than a ton. The 
strength is dependent on the size of the iron core, the 
quality of the iron, the amount of wire in the helix, and 
the strength of the magnetizing current. 

Core. — The core can be. magnetized only to the point 
of saturation, beyond which increase in size of helix or 
strength of current can produce no increase of magnetic 
strength; hence its mass should be duly proportioned 
to that of the helix, hollow cores, of sufficient mass, 
having the same efficiency as solid ones. Its ends 
should project beyond those of the helix. The iron 
should be soft and homogeneous in structure to render 
it capable, not only of the highest degree of magneti- 
zation, but of rapidly acquiring or losing its magnetism 
at the closing or opening of the electric circuit ; a 
quality on which the practical value of the electromag- 
net largely depends. 

Coefficient of Magnetic Induction. — This property of 
magnetic permeability, or conductivity for the lines of 



ELE C TR OMA GNE TISM. 7 7 

force, is termed the coefficient of magnetic induction, and is 
found in various bodies in different degrees, but in none 
to the same degree as in soft iron, which is therefore 
said to have a high coefficient of magnetic induction. 

Helix. — The helix may consist of fine or of coarse 
wire of any conductivity, copper being practically the 
best, and the total volume of current with a given mass 
of wire may be the same in either case, while the resist- 
ance may vary greatly. A helix of ten coils of wire of 
a given cross-section and length may equal in mass 
another helix of a hundred coils of one tenth the cross- 
section and ten times the length: and resistance varying 
directly as length and inversely as cross-section, and 
current, with a given E, M. F., varying inversely as re- 
sistance, the volume of current in any given section of 
the fine wire would be only one tenth of that in a 
coarse wire of equal length, while the total volume in 
the mass would be the same in either case, since the fine 
wire has ten times the number of coils. 

The rule is to make the resistance of the helix equal 
to the external resistance of the current, and thus adapt 
the magnets to the conditions of the work for which 
each is designed. 

The diameter of the coils, wuthin certain limits, does 
not affect the strength of the magnet, since the field of 
magnetizing electric force surrounding the wire varies 
directly in area and inversely in magnetic effect as the 
square of its distance from the core, variation in one 
sense compensating variation in the other; since its 
strength at any point in the larger area, equally dis- 
tant from the wire, is the same as in the smaller area, 
while the distance of the wire from the core is propor- 
tionally greater. 

With such proportion between the mass of the helix 
and of the core as to insure saturation without excess. 



78 DYNAMIC ELECTRICITY AND MAGNETISM. 



the diameter of the helix should not exceed one half its 
length. 

Electromagnetic Saturation. — A perceptible amount of 
time is required to produce magjietic saturation of the 
core, which, in the case of very large magnets, may 
amount to two seconds. It has also been observed that 
this result is attained more rapidly with high E. M. F. 
coupled with high resistance than with low E. M. F. 
and low resistance, though the volume of current in 
each case is the same. 

Form of Electromagnets. — The horseshoe or U form, 
shown in Fig. 32, in which both 
poles attract the same armature, 
has, as in the steel magnet, the 
greatest practical efficiency. The 
winding must be in the same di- 
rection in both coils, as if a 
straight bar were thus wound 
and then bent; which requires 
the wire to cross to the opposite 
side at the bend as shown. 
A modification of this form is seen in the rectangular 
form shown in Fig. 2iZ^ t^^e wire may also be wound on 
separate bobbins and slipped over the cores as shown. 

Armature. — The armature should be of the best soft 
iron, and of such form and mass as 
to embrace the greatest practicable 
number of lines of force, since it 
is itself a magnet during contact, 
and its force, whatever it may be, 
varies in the same ratio as the mag- 
net's force, and the portative force 
equals the sum of the two. Hence if the magnet's 
force equals x and the armature's force j, the por- 
tative force is x -\- y\ and if the magnet's force is 




Fig. 32. 




Fig. 33. 



RLE C TROMA GNE TISM. 



79 



doubled, the portative force becomes 2{x +7); if halved, 

X -\- y 

; the proportion being the same for any other 

variation. 

The magnetization of the steel magnet can be accom- 
plished most efficiently by the electromagnet. 

Experiments in Diamagnetism. — The electromagnet, by 
its superior power, affords the means of examining dia- 
magnetic bodies, not practicable with the steel magnet. 
For this purpose Faraday, in 1845, used the apparatus 




Fig. 34. 

shown in Fig. 34, which consists of two powerful electro- 
magnets, A and B, having hollow cores, between whose 
opposite poles the body under examination may be 
suspended, as shown; the distance between the poles 
being adjusted as required by adjusting the movable 
frames FF, to which the cores are attached, to the re- 
quired position, as indicated by the scale i?^, where 
they are secured by the binding-screws FF. The poles 
terminate in cone-shaped armatures, attached by screws, 
by which the magnetic force is concentrated between 
the rounded points. 



8o DYNAMIC ELECTRICITY AND MAGNETISM. 

If the body under examination is repelled from the 
concentrated magnetic field thus formed, it is classed as 
diamagnetic ; if attracted, as f?iagnetic, or pa7'amagnetic. 
The tests are made either with small balls of the vari- 
ous substances, suspended near the poles, which are 
brought into close proximity, or with straight bars 
suspended between the poles, when placed farther apart. 
The balls are repelled or attracted as above, according 
as they are diamagnetic or paramagnetic, while the bar, 
if diamagnetic, sets itself equatorially, as shown by the 
bar ab, Fig. 34, but, if paramagnetic, axially, that is, 
parallel to a line joining the poles. 

The reason of this becomes obvious when it is con- 
sidered that the magnetic field, as has been shown, con- 
sists of magnetized matter, which may be air at the 
ordinary density, or rarefied to any extent possible by 
the formation of a partial vacuum, or some other 
gaseous body, as oxygen; and the suspended body be- 
comes itself a part of this field when attracted into it. 
Hence if paramagnetic, like iron, it can form within 
itself a field which may equal or even greatly exceed in 
strength that of the gaseous body in which it is sus- 
pended, and hence is drawn into that position in which 
it can embrace the greatest number of lines of force, 
which, in a straight bar, is the axial position; but if dia- 
magnetic, like bismuth, it is pushed aside by the exist- 
ing lines of force, from the stronger to the weaker part 
of the field, where the lines of force are equal to its re- 
ceptive or magnetic inductive capacity, which in the 
case of a straight bar is the equatorial position, where 
the greater part of the bar is most remote from the cen- 
tral region of greatest magnetic intensity. Hence /^r^- 
magnetic bodies are those in which magnetic inductive capacity 
is high, dia?nagnetic bodies those ifi which it is tow. 

Such a test is not to be regarded as indicating the 
absolute diamagnetism or paramagnetism of the body 



ELECTROMAGNETISM. 8 1 

under examination, but as a comparison with the mao^- 
netic condition of the gaseous medium occupying the 
field, which would ordinarily be the air at its normal 
density; the test being analogous in this respect to that 
for the specific gravity of bodies by a comparison of 
their weight in air with their weight in water. 

Gases are tested by means of bubbles inflated with 
them showing attraction or repulsion; and liquids simi- 
larly, when suspended in glass vessels; but a correction 
is evidently required in the first case for the matter com- 
posing the walls of the bubble, and in the second for 
the glass of the vessel, since the magnetic condition of 
either might differ widely from that of the gas or liquid 
and seriously affect the result. 

List of Diamagnetic and Paramagnetic Substances. — The 
principal substances found to be diamagnetic are as fol- 
lows : bismuth, phosphorus, antimony, zinc, mercury, 
lead, silver, copper, gold, water, alcohol, tellurium, se- 
lenium, sulphur, thallium, hydrogen, air. The princi- 
pal ones found to be paramagnetic are as follows: iron, 
nickel, cobalt, manganese, chromium, cerium, titanium, 
oxygen; also substances containing the above metals in 
combination. The proper magnetic classification of 
platinum is not settled; it has been assigned to each list 
by different observers, the weight of evidence being in 
favor of its paramagnetic character; but, when chemi- 
cally pure, Wiedemann considers it diamagnetic. 
Flames, smoke, and hot air tend to move, in the mag- 
netic field, from higher to lower potential, which would 
indicate that they are diamagnetic; but this is not clear, 
since the movement may be due to the convection of 
the air and its diamagnetism. It has been observed that 
bismuth, when pulverized, made into a paste with 
mucilage, and formed into a roll, sets itself equatorially 
in the magnetic field, like a bismuth bar; but when com- 



82 DYNAMIC ELECTRICITY AND MAGNETISM. 



pressed into a flat plate its position becomes axial, an 
effect attributed to the semi-crystalline structure of the 
mass. 

Deflection of the Electric Current by the Magnet. — It has 
been shown that a magnetic needle or bar magnet free 
to rotate takes position at right angles to a fixed wire 
bearing an electric current; conversely it may be shown 
that if the wire be free to rotate it will take position at 
right angles to a needle or bar magnet in a fixed posi- 
tion, a result which evidently follows from the law of 
action and reaction. 

Ampfere's Table. — This can be explained with the ap- 
paratus devised by Ampere, shown in Fig. 35, known 
as Ampere's table^ which consists of a wire bent as 

shown, and suspended 
so as to rotate horizon- 
tally round its centre 
of gravity; its ends dip- 
ping into mercury cups 
to insure perfect con- 
tact; and having arms 
and supporting stand- 
ards of brass, with 
which the battery wires 
which supply the cur- 
rent connect at bottom, 
as shown, so that the 




Fig. 35. 



wire coil becomes part of the circuit. 

This coil will adjust itself with its plane at right 
angles to the length of a bar magnet thrust into it; its 
position being reversed when the poles or the current 
are reversed, and similar effects but weaker being ob- 
served when the magnet is held above or beneath the 
coil. 

If the magnet be entirely withdrawn the coil will 



ELECTROMAGNETISM, Sj 

assume the same position with reference to the earth's 
magnetism to that which it would assume if a magnet 
were placed beneath it with its - south-seeking pole 
turned north, so as to represent the earth's magnetism. 
The plane of the coil will then be at right angles to the 
magnetic meridian, and that face turned north which 
would be indicated by the left hand of Ampere's little 
figure swimming with the current, face downward, at the 
bottom of the coil. 

The Solenoid. — If this single coil be replaced by the 
solenoid represented by Fig. 36, which consists of a helix 
with straight portions of the wire returned to the cen- 
tre as shown, the mag- 
netic effect is greatly in- 
creased, each convolution 
assuming the same posi- 
tion as the single coil, so 
that the solenoid takes 
the same position as the 
magnetic needle and has 
north and south polarity. This might still be accounted 
for by the current's reaction setting the planes of the 
coils at right angles to the magnetic meridian in obedi- 
ence to the earth's magnetism, but it is found that the 
poles of the solenoid are repelled by like poles of the 
magnet and attracted by unlike, which indicates that it 
has true magnetic properties like those of the needle, 
which is made further apparent by the fact that two 
solenoids suspended in mutual proximity behave like 
two needles similarly placed, exhibiting polar attraction 
and repulsion in the same manner. 

Here, then, we have a current-bearing wire, which may 
be copper or any other metal, behaving like a steel mag- 
net; which furnishes strong proof, in addition to that 
already adduced, of the close affinity, if not actual 



Fig. 36. 



^4 DYNAMIC ELECTRICITY AND MAGNETISM. 

identity, of electricity and magnetism. It is also found 
that an electric current flowing in rarefied air as its 
medium, in a glass tube in which the highest attainable 
vacuum has been produced, is attracted or repelled by a 
magnet like the current in the wire. 

The polarity of the solenoid is much weaker than 
that of the magnetic needle, but may be greatly reen- 
forced by placing within it a soft-iron core, which in 
fact makes it an electromagnet. It now takes position 
in the plane of the magnetic meridian with the energy 
of the needle, but its polarity must be ascribed chiefly 
to the magnetism of the core rather than to that of the 
current, as shown in the solenoid without a core. 

De La Rive's Floating Battery. — De La Rive used for 
the above experiments a little floating battery. It is 
easily constructed with little plates of zinc and carbon, 
or copper, attached to a large flat cork and floated on 
water acidulated with sulphuric acid; a light coil or 
solenoid attached to the plates projecting from the upper 
surface of the cork. It is simpler, cheaper, and more 
easily constructed than Ampere's table, but less effect- 
ive and limited to a smaller number of experiments. 

Mutual Induction of Electric Currents. — By using the 
rectangular frame represented by Fig. 37 with Ampere's 
table, the mutual attraction and repulsion of electric 
currents may be shown. If a current 
is flowing round the frame, as shown, 
and a straightened section of another 
current-bearing wire be held in close 
} proximity, parallel to either of its 
vertical sections, the frame wire will be 
attracted if the two currents flow in the 
Fig. 37. same direction^ but repelled if they flow in 

opposite directions j this will also be true when the wires 
are inclined to each other at an angle, attraction taking 



jW^ 




ELE C TR OMA GNE TISM. S 5 

place when the currents flow either toward or fro7n a com- 
mon point, but repulsion whefi one flows toward and the other 
from a comnion point. 

Ampere, who discovered this mutual action, gave to 
it the name of electrodynamics. Its explanation may be 
found in the mutual action of solenoids already de- 
scribed, whose like poles were shown to repel and unlike 
to attract, like the corresponding poles of the magnet. 
It is evident that when two like solenoid poles are 
brought into mutual proximity, the currents in adja- 
cent sides of their coils must flow in opposite direct 
tions, one upward and the other downward; hence the 
inductive lines of force in the field surrounding the two 
poles meet in opposition and repel each other as in 
steel magnets, as already explained. But if unlike solen- 
oid poles be brought into proximity the currents in ad- 
jacent sides flow in the same direction, and the lines of 
force interlock and draw the poles together, as in steel 
magnets. Now if the adjacent sections of the solenoid 
coils in proximity be straightened, we have exactly the 
same conditions as in the rectangular wire frame and 
straightened section of wire in proximity, and the re- 
sults in each case are the same. 

The wires and surrounding air in all these cases are 
the media through which energy manifests itself by 
molecular action; and we call these manifestations elec- 
tric, or magnetic, or electromagnetic, according to the 
nature of the media, and the conditions under which 
the manifestation occurs. 

When a current flows through a conductor, it is found 
that the effect of induction is to produce an opposite 
current in any adjacent parallel conductor. 

The nature of this action may be represented by the 
following diagram: 

(«) + 10 + + + + + + + + + I 

(b)- , ^ 



S6 DYNAMIC ELECTRICITY AND MAGNETISM. 

Let a represent a conductor in which a current is 
flowing from left to right by virtue of the difference of 
potential represented by -f- lo at the left and + i at the 
right; and let b represent an adjacent parallel con- 
ductor. Since inductive influence radiates from a 
charged body equally in all directions, only a small 
fraction of the lines of electric force radiating from a 
are intercepted by b ; but for convenience we may rep- 
resent this fraction by — -^at the right, and — 2 at the 
left. Now, since, as has been shown, the positive pro- 
duces by induction an equal corresponding negative, 
and vice vei'sa^ the positive potential in a, represented by 
-\- 10, induces, under the conditions named, a negative 
potential in b, represented at the adjacent point on the 
left by — 2, while on the right -\- \ \xv a induces—^ in 
b\ and since electric movement is always from higher 
to lower potential, the current induced in b must flow 
from right to left opposite to that in a. 

Hence when currents in two or more adjacent paral- 
lel conductors flow in the same direction the effect of 
their mutual induction is to produce in each a counter- 
current, which reduces the volume of the primary cur- 
rent; the effective current by which useful work is ac- 
complished being then represented by the difference be- 
tween them. As in the illustration, if the primary 
current were represented by 10 and the induced oppos- 
ing current by 2, the effective current would be repre- 
sented by 8. 

But if the primary currents flow in opposite direc- 
tions, the effect of induction is reversed, and the volume 
of effective current in each conductor increased, as can 
easily be seen by the following diagram: 

(^)+io + + + + + + + + +i 
(^)+ 1 + + + + + + + + +10 
In which the current in c flows from left to right, and in 
^/ from right to left. The positive potential 10, at the 



ELECTROiVA GNETISM. 



^1 



left of r, induces at the same end of ^ a much stronger 
negative than the potential i of ^ can induce in c ; con- 
sequently the difference of potential between the oppo- 
site extremities of d must be greater than if c were re- 
moved, and hence its E. M. F. and resulting volume of 
current must be greater. And the same effect in c must 
follow from the difference of potential at the right be- 
tween the lo of d and the i of c. 

Rotary Movement by Current Induction. — Fig. 38 repre- 
sents an apparatus by which the continuous rotary 




Fig. 38. 

movement of a wire maybe produced by current induc- 
tion. E F represents a circular copper trough con- 
taining mercury, round which is wound a coil of insu- 
lated coper wire, seen through the opening near in^ its 
terminals being connected with the binding-posts m 
and o. 

From the post n^ which is connected with m^ a wire 
leads to the central insulated brass post A^ on which is 
pivoted, in a mercury cup, a wire whose ends, B and C, 
dip into the mercury in the trough, with which the post 
p is connected as shown. 

The post o is connected with the positive pole of a 
battery, and/ with the negative; the current therefore 



88 DYNAMIC ELECTRICITY AND MAGNETISM. 

flows from o through the coil to m, in a direction oppo- 
site to that in which watch-hands move, thence through 
n to the post Ay which it ascends, and dividing, passes 
down the arms B and C, and through the mercury and 
trough to/. 

It is evident, then, that the branch current flowing 
down C, and that in the coil flowing from o to F^ both 
flow toward the common point F^ and hence attract 
each other, while beyond that point the coil current 
^ovjs from 7^ and the current in C toward it, and hence 
repel each other; hence C, being free to move, is im- 
pelled toward the observer. In like manner it can be 
shown that B is impelled /r^/;? the observer; so that the 
wire rotates round the trough in the same direction 
as watch-hands move, and opposite to the direction of 
the current in the coil. 

By connecting n with o instead of with m^ and chang- 
ing the positive battery wire from o to m, the current in 
the coil is reversed and the rotation of the wire reversed 
also. 

Water acidulated with sulphuric acid may be sub- 
stituted for the mercury in the trough, and the lower 
part of the wire, which is immfersed in the fluid, may 
also be extended by bending it into a circular form so 
as to conform to the shape of the trough, and furnish a 
better conductor than the fluid alone. 

Rotation may also be produced in mercury or dilute 
acid, contained in a circular vessel placed above or be- 
neath either pole of a strong magnet, if the vessel be 
connected with one pole of a battery while a wire from 
the other pole dips into the center of the fluid, so that 
the current flows radially either from or toward the 
center. 

A magnet also, loaded so as to float vertically in a 
vessel of mercury, or pivoted in any other convenient 



ELECTRON AGNETISM. 89 

manner, will rotate round its vertical axis, if a battery 
wire dips into a mercury cup at its upper end while the 
other pole of the battery connects with the outer edge 
of the mercury in the vessel. 

These experiments may be varied indefinitely, but are 
all explainable by Ampere's rules for the mutual induc- 
tion of currents already given; and those with the mag- 
net indicate that the lines of force in the field around 
its poles are of the same nature as those around a cur- 
rent-bearing wire. 

Ampere's Theory of Magnetism. — From observation of 
various electromagnetic phenomena, such as we have 
been considering. Ampere deduced the theory that 
electric currents are in constant circulation around the 
molecules of all bodies capable of acquiring magnetism, 
so that every such molecule is the centre of a field of 
electric force; and that when the body is unmagnetized 
these currents circulate in different directions, and 
hence no external effect is produced; but that under the 
magnetizing influence they are all made to circulate in 
the same relative direction, and hence between the ad- 
jacent sides of any two molecules they are in opposition 
and neutralize each other; and as this must be true of 
every interior molecule on all its sides, all interior 
circulation must cease; while the currents from the ex- 
terior sides of the surface molecules, having no oppo- 
sition, and all now circulating in the same direction 
round the mass at right angles to its length, produces 
the effect called magnetism. 

In tempered steel the currents retain their harmony 
of movement after the magnetizing influence is with- 
drawn, but in soft iron they resume the irregular move- 
ment and the magnetism disappears. 

This theory, so far as it relates to the external or field 
currents, is in harmony with the theory of moleculaf 



90 DYNAMIC ELECTRICITY AND MAGNETISM. 

motion already given; while the latter accounts more 
satisfactorily for the origin of the currents, their per- 
manency in tempered steel and want of permanency in 
soft iron, by ascribing them to the motion of the mole- 
cules themselves, and their change of position under 
the magnetizing influence. Ampere's theory gives an 
effect without an adequate cause, while, if molecular 
motion be regarded as a natural condition of all bodies, 
which seems to be w^ell established, we have, in its 
various forms, ample cause not only for those manifes- 
tations of energy which we term magnetism and elec- 
tricity, but also for other kindred physical phenomena. 

Generation of Electric Currents by Induction. — In 1831 
Faraday made the important discovery that an electric 
current can be induced in a conductor forming a closed 
circuit by the movement of a magnet in proximity to 
it, and also that the same effect can be produced by the 
similar movement of a current-bearing conductor; both 
results furnishing additional proof of the close affinity 
of electric and magnetic induction and of the correct- 
ness of Ampere's theory of the circulation of electric 
currents round the magnet. 

Current Induced by Magnet. — These results may be 
verified in the following manner: Let the terminals of 
a hollow wire coil be connected with an astatic galvano- 
meter, as shown in Fig. 39; on the insertion of a bar 
magnet quickly into the interior, the needle will be de- 
flected, showing that an electric current has been induced 
in the coil. This current is only transient, continuing 
only during the downward movement of the magnet; 
but when the magnet is withdrawn the needle is de- 
flected in the opposite direction, showing the induction 
of a reverse current which continues only during the 
upward movement. If the poles of the magnet be re- 



ELECTR OJ/A GXE TISM. 9 1 

versed, the direction of each current will also be re- 
versed in accordance with Ampere's rule. 

Similar results may be obtained by placing a soft 




Fig. 39. 

iron core within the coil, and the alternate approach 
and withdrawal of each pole of the magnet from its 
projecting end. 

Current Induced by Another Current. — For the current- 
bearing conductor a small coil connected with a battery 
may be used as shown in Fig. 40. This is known as the 
primary coil and fits inside of the other, which is known 
as the secondary. On its insertion a current is induced 
in the secondary, the reverse of that in the primary, and 
on its withdrawal, a direct current, the same as that of 
the primary, is induced. 

If the induced effect be regarded as magnetic, then in 
the experiment with the magnet it is evident that on 
the insertion of either pole the opposite polarity is in- 
duced in the coil, so that the induced current flows in 
the reverse order to that which would produce the ex- 
isting polarity of the bar, but on its withdrawal the 



92 DYNAMIC ELECTRICITY AND MAGNETISM. 

opposite effect is produced, and the induced current 
is direct, the same as would produce the existing polar- 
ity of the bar. 




Fig. 40. 

The same would be true of the experiment with the 
primary coil, which may be regarded as another form of 
bar magnet with currents circulating in its coils in a 
similar manner to those which, as shown by Ampere, 
circulate in the field of the magnet ; hence we have the 
inverse current on insertion, as has been shown, and the 
direct current on withdrawal. 

On the insertion of either the magnet or primary coil a 
varying number of lines of force is cut by the secondary 
coil, and this creates a difference of potential between 
that part of the secondary coil which cuts the lines and 
that which does not, which becomes an important fac- 
tor in the generation of the current ; such generation 
being always a result of difference of potential. 

It will also be noticed that the inverse current is in- 
duced when the movement is such as to produce increase 
in the number of lines cut, and the direct current when 
it is such as to produce decrease; the force of the induced 



ELECTR OMA GNE TISM. 



93 



current being always in such direction as to oppose the 
mechanical movement by which it is produced, accord- 
ing to what is known as Lenz's law. 

Current Induced by Opening or Closing Primary Circuit. 
— If a primary coil be placed inside of a secondary and 
insulated from it, electric currents can be induced in the 
secondary circuit by opening or closing the primary cir- 
cuit. Let the apparatus be arranged as shown in Fig. 
41 ; the secondary coil connected with the galvanometer 




Fig. 41. 

G, and the primary with the battery P ; the latter 
connection being made, for convenience, through the 
mercury cups ^ ^'. No movement of the needle occurs 
while the connections remain undisturbed, but if the 
primary circuit be opened by lifting a wire terminal 
from one of the mercury cups a deflection of the needle 
occurs, indicating a transient current in the secondary 
circuit in the same direction as that in the primary ; on 
closing the primary circuit an opposite deflection of the 
needle indicates a transient current in the secondary 
circuit the reverse of that in the primary. By an ex- 
periment similar to this, Faraday made the discovery of 
current induction as stated. 

The terms " make'" and " break'' are used for conven- 



94 DYNAMIC ELECTRICITY AND MAGNETISM. 

ience, to denote respectively the opening and closing of 
a circuit in this manner. 

Current Induced by Varying^ the Strength of Primary 
Current. — If a short circuit be arranged by a wire mak- 
ing direct connection between the mercury cups, by 
which a portion of the current can be diverted from the 
primary coil, then, on closing this short circuit, the pri- 
mary current is weakened and a transient, direct current 
induced in the secondary coil ; but on opening it, so that 
the full current again flows through the primary coil, a 
transient reverse current is induced in the secondary coil. 

Here also it is seen that the inverse current results 
from increase of inductive influence, and the direct from 
decrease. 

Results of Current Induction. — These results may be 
summarized as follows : i. A magfzet, moving i7i close proxi- 
mity to a conductor forjning a closed circuity induces in it an 
electric current which continues during the movement^ and is 
the reverse of that which would have produced the magnef s po- 
larity when the movement is such as to increase the inductive in- 
fluence, but the same as would have produced the magnef s 
polarity when the movement is such as to decrease the induc- 
tive influence. 

2. A current-bearing conductor, moving in close proximity 
to a conductor forming a closed circuit, induces in it an elec- 
tric current, which continues during the movement, and is the 
reverse of the primary current when the movement is such as 
to increase the inductive influence, but in the same direction 
as the primary citrrent when the movement is such as to de- 
crease the inductive influence. 

3. A cur re7it-b earing conductor, placed in close proximity 
to a conductor forming a closed circuit, induces in it, wheji the 
pri??tary circuit is opened, a transieftt electric current in the 
same direction as the primary current, aiid an inverse tran- 
sient current when the primary circuit is closed. 



ELE C TR OMA GNE TISM. 9 5 

4. A current-hearing conductor, placed in close proximity 
to a conductor fortJiifig a closed circuit, induces in it, when 
the primary circuit is weakefied, a transient electric current 
in the same direction as the primary current, and an inverse 
transient current when the primary current is strengthened. 

Generation of Current Dependent on Variation of Inter- 
cepted Magnetic Force. — The E. M. F. generated by these 
mechanical movements varies as the number of lines of 
force cut by the conductor per unit of time, and the 
strength of the induced current varies as the E. M. F. 
divided by the resistance of the circuit. But there must 
always be a difference in the number of lines of force 
cut by different parts of the conductor, whether it be 
moved with reference to the field or the field with refer- 
ence to it, in order to produce difference of potential. 
No current can be induced in a conductor moving par- 
allel to itself in a perfectly uniform field, since there is 
no difference of potential ; but if the field vary in uni- 
formity, or different parts of the moving conductor, in 
a uniform field, cut a varying number of lines of force, 
there is, in either case, generation of current as a conse- 
quence of difference of potential. This development of 
the original discovery is also due to Faraday, and its 
application in the construction of electromagnetic ap- 
paratus for practical use is of the highest importance. 

The transient electric currents can, by various de- 
vices, be produced in such rapid succession as to be 
practically continuous, and either direct or alternating 
as required. 

Coefficient of Mutual Induction. — The inductive energy 
of a current-bearing conductor is found to vary as the 
area which it is capable of inclosing and as the strength 
of the current which it carries ; and the mutual induc- 
tive action of such conductors on each other, as repre- 
sented by the number of lines of force intercepted in 



96 DYNAMIC ELECTRICITY AND MAGNETISM. 



1 



each by the other, is known as the coefficient of mutual 
induction; and this quantity must attain its relative 
maximum in such conductors when each intercepts the 
greatest possible number of lines of force in the others, 
which must evidently be when they are in the closest 
relative position. But its absolute value, with unit cur- 
rent, must vary as the total area capable of being in- 
closed by the conductors which, in a coil, is represented 
by the number and diameter of the convolutions. 

But since mutual induction is only one of the con- 
ditions of the generation of E. M. F. by the coil, it must 
not be understood that siich generation varies as this 
quantity alone, but is dependent on a concurrence of 
the conditions already mentioned. 

Self-induction. — A current-bearing conductor having 
the form of a coil, or any form in which different parts 
of the same circuit are brought into mutual proximity, 
experiences a momentary inductive effect on the open- 
ing or closing of the circuit similar to that produced in 
a secondary coil. 

This, as in the secondary coil, induces a transient cur- 
rent in the same direction as the original current on 
opening the circuit, and in the opposite direction on 
closing, which is known as the extra current. In a 
straight conductor, no part of which returns so as to be 
adjacent to another part of itself, there is still a slight 
self-induction, which, however, is of little practical con- 
sequence ; but in coils, the close proximity of the nu- 
merous turns greatly increases it. The slight self-in- 
duction noticeable in a straight conductor may result 
from its being produced in an interior shell, as repre- 
sented in cross-section, by the original current in the 
outer shell. 

Extra Current. — It is evident that the inverse extra 
current, induced on closing the circuit, must neutralize 



RLE C TR OMA GNE TISM. 97 

the original current to the extent of its strength, while 
the direct, induced on opening, adds its strength to the 
original ; hence it is found that when a primary coil is 
inclosed within a secondary, the transient current in- 
duced in the secondary on opening the primary circuit 
is much stronger than that induced on closing it. The 
extra current at " make" retards the maximum effect 
for an infinitesimal fraction of a second, so that the 
transient current induced in the secondary does not 
attain its maximum till just before it ceases ; and in 
like manner, at " break," the maximum is not attained 
till an instant after the break occurs, and just before 
the transient current disappears. 

The case is analogous to the flow of water in a trough 
under the influence of gravity ; the inertia of the mass 
retarding the current when it begins, but continuing it, 
with accumulation of water in the trough, when sud- 
denly interrupted. 

The Spark. — The high E. M. F. of the primary current 
at " break" produces an electric spark between the ter- 
minals of the secondary coil when separated by an air- 
space, which varies from a fraction of an inch to several 
inches, and is similar tc that which occurs between the 
poles of a static electric machine. No spark occurs at 
" make" except in very large coils, and even in these it 
is feeble, indicating clearly a much lower E. M. F. than 
at "break," as already explained. 

Induction of Core. — It has been shown that the mag- 
netic energy of a solenoid is greatly increased by the 
insertion of a soft-iron core, and reciprocally, that in 
the electromagnet the core acquires its magnetism from 
the electric current in the inclosing coil. The core, in 
either case, forms a medium of high conductivit]' of 
magnetic permeability greatly superior ta thaV of the 
air which it displaces, and its magnetism reacts as a 



9B DYNAMIC ELECTRICITY AND MAGNETISM. 



coefficient of the magnetism and electricity of the in- 
closing coil ; so that the magnetic and electric strength 
developed is in proportion to the coefficient of mag- 
netism in the core, as this property of permeability has 
been termed. 

Induction-Coil. — The principles of current-induction 
thus developed have given rise to the apparatus known 
as the induction-coil, or inductorium as it is also called, 
the construction of virhich, as improved, is due to Ruhm- 
korff. 







Fig. 42. 



Its general construction, with some variation in minor 
details, will be understood from Fig. 42. It consists of 
a primary coil of a few layers of cotton- wound, coarse 
copper wire, usually No. 14-20, inclosing a core made of 
a bundle of soft-iron wires. This coil is inclosed in a 
secondary coil of very fine, silk-wound copper wire, 
usually No. 26-30, several hundred times the length of 
the primary, from which it is insulated by a hard-rubber 



ELECTROMAGNETISM. 99 

tube. On account of the extreme fineness of this wire, 
great care is required in the winding, to have the layers 
perfectly even, and to avoid a cross or a break. A coat- 
ing of paraffine is applied during the winding to improve 
the insulation, and the layers are separated by paraffined 
paper. 

Condenser. — The coil thus completed is mounted on a 
base of wood or hard rubber, in the bottom of which is 
placed a condenser consisting of a number of sheets of 
tin-foil insulated from each other by paraffined paper, 
the alternate ends of the foil projecting beyond the 
paper at each end, so that all the oddly numbered sheets 
are in contact at one end, and all the evenly numbered 
sheets at the other end ; and each end is connected with 
an interior terminal of the primary coil, the exterior 
terminals of which are connected with the battery; so 
that the condenser is directly in the primary circuit. It 
is sometimes omitted from small coils. 

Interrupter. — An interrupter, or contact-breaker, is also 
placed in the primary circuit, mounted on one end of 
the base, and varies in construction according to the 
size and design of the coil. In small coils it consists of 
a light steel spring known as the vibrator, carrying at 
one extremity a small armature placed opposite the 
projecting end of the core, which closes the circuit by 
the pressure of a platinum plate, attached to it, against 
a platinum point fixed in the end of a set-screw; but on 
the passage of the current the magnetism of the core 
attracts the armature and opens the circuit; the cessa- 
tion of the current demagnetizes the core, and releases 
the armature, which, being forced back by the spring, 
again closes the circuit as before. The set-screw regu- 
lates the amplitude of the vibration, and a second set- 
screw is sometimes used to regulate the tension of the 
spring. 

LOFC. 



100 DYNAMIC ELECTRiClrV AND MAGNETISM. 

The make and break thus produced occur with great 
rapidity, giving rise to a series of transient, alternating 
currents, induced in the secondary coil, which thus be- 
come continuous, and are known as l\\^ faradic current. 
If a weak faradic current is desired, the amplitude of 
the vibrations is reduced, and if necessary the tension 
of the spring also, so that the primary circuit is opened 
or closed before the transient current in each case has 
attained its maximum ; but if a strong faradic current 
is desired, the amplitude of the vibrations and tension 
of the spring are increased and the opposite effect pro- 
duced. 

The vibrator can be used for coils giving sparks 17 
to 28 inches in length, but in very large coils the extra 
current at break produces a series of sparks between 
the points of contact which are liable to melt the plati- 
num point, injure the coil by heating, and interfere with 
the promptness of the break by their conductivity ; 
hence interrupters not liable to this defect are required. 
The one shown in Fig. 42 is Foucault's, and consists of 
a brass lever Z, supported on a vertical spring, its left 
end carrying an armature, shown just above the pro- 
jecting end of the core, and its right end a plunger 
tipped with platinum, which dips into the mercury in 
the cup J/, the surface of which is covered with alcohol. 
The attraction of the armature lifts the point out of the 
mercury, and the alcohol, being a non-conductor, pre- 
vents the spark, the contact being renewed again h^ 
the force of the spring. The adjusting screws shown 
regulate the amplitude of the movement for currents of 
different strength. 

Interrupters operated by clock-work, or by some ex- 
ternal force, are also used ; and when sparks alone are 
required at intervals, they may be operated by hand. 
One used by Spottiswoode consists of a brass wheel 



ELECTRO MA GNE TISM. I O I 

having a number of radial slots filled with hard-rubber, 
and rotated by a little engine ; a platinum spring presses 
on the circumference, opening the circuit when in con- 
tact with the rubber, and closing it when in contact 
with the brass. By this means the make and break can 
be produced with great rapidity, and the smoothness of 
the induced current proportionately increased. 

Sliding Core. — It is often desirable, especially in coils 
for medical use, to vary the strength of the induced 
current to a greater extent than can be done by varying 
the amplitude of movement in the interrupter ; this 
may be done by varying the magnetism in the primary 
coil by sliding the core in or out to any extent required 
for the variation. In such case a small electromagnet, 
placed in the primary circuit, is used to operate the 
interrupter instead of the magnetism of the core. 

Water Rheostat. — The same object may be accom- 
plished by varying the resistance of the primary circuit; 
which can be done by connecting it with a column of 
water contained in a vertical glass tube set on the base; 
one terminal being let into the tube at bottom, while 
the other is attached at top to a plunger, by which the 
distance between the terminals, and consequently the 
resistance, can be varied as desired ; the plunger having 
sufficient sliding friction to retain it at any point. The 
term rheostat is used to designate any apparatus thus 
used for resistance, this particular kind being known as 
the water rheostat. 

Construction of Core. — A bundle of wires is preferred 
to a solid bar for the core, since, as in laminated mag- 
nets, it prevents the formation of Foucault currents by 
giving the lines of force a normal direction toward the 
poles. They should be bound together by soldering 
the ends or otherwise, so as to be moved as one mass. 



102 DYNAMIC ELECTRICITY AND MAGNETISM. 

and to furnish an even surface at the end next the inter- 
rupter for its proper adjustment. 

Operation of Condenser. — The object of the condenser 
is to absorb the extra current which occurs at " break," 
and thus prevent the spark which is liable to occur at 
the interrupter, and which interferes with the sudden- 
ness of the break by its conductivity ; this absorbed 
current opposes the reverse current induced at " make" 
and retards its maximum effect. Hence the effect of 
the condenser is an increased promptness of action at 
" break," and a decrease in promptness, or partial sup- 
pression, at " make/' so that the induced current at 
" break " becomes practically the current of efficient 
work. 

Leyden Jar as a Condenser. — An insulated Leyden jar 
or Leyden battery may be used as a condenser in the 
secondary circuit, to increase the energy of the spark, by 
connecting its coatings respectively with the opposite 
terminals; knobs from the opposite coatings being 
brought within sparking distance, a series of sparks of 
greatly increased energy passes between them, similar 
to those which pass between the opposite poles of a 
Holtz or a Topler machine ; a result which is proof of 
the identity of static and dynamic electricity. 

Special Construction. — In large coils special methods 
of winding and insulation are required on account of 
the high induction and potential difference between the 
primary and secondary coils and between different parts 
of the secondary. This is necessary to prevent short- 
circuiting and permanent injury by burning, in case a 
spark, from insufficient insulation, should cross between 
the coils or between the layers of the secondary ; also 
to bring the coils into the best inductive relation, and 
to reduce what is known as the " Leyden jar effect " 
which occurs between the outer coating of the primary 



ELECTR OMA GNE TISM, 



103 



and inner coating of the secondary, and interferes with 
electric action. 

It is found that in the primary coil induction is great- 
est at the centre and least at the ends; also that the 
potential difference in the secondary is least at the cen- 
tre and greatest at the ends ; hence in large coils the 
insulation between the primary and secondary, which 
should always be ample, is often adapted to these con- 







a) 



A Fig. 43. B 

ditions by making the insulating tube thicker at the 
ends than at the centre, as shown at A, Fig. 43. This 
gives increased insulation at the ends where it is most 
required, and brings the greater mass of the secondary 
coil to the centre where induction is the strongest. 

The same results may be accomplished in a different 
manner, as shown at B, by making the insulating tube 
of the same diameter throughout, with projecting ends 
for greater end insulation, and winding the secondary 
coil in an elliptical form with the greater mass of wire 
at the centre. The primary coil in both cases occupies 
the interior of the tube. 

It is evident that the potential difference must be much 
greater between adjacent layers, especially at the ends, 
than between adjacent convolutions, since each convolu- 
tion adds its quota to this difference, and each layer 
being doubled back on the one next underneath, the 
convolutions of each pair thus formed are separated at 
each ahernate end from those of the next adjacent paii 



104 DYNAMIC ELECTRICITY AND MAGNETISM. 

by the whole number of convolutions in the two layers. 
This produces, in large coils which require long layers, 
too great a strain on the insulation at those points ; 
hence such coils are usually wound in two or more sec- 
tions, as shown at B, insulated from each other by hard- 
rubber, through which the wire passes to connect them, 
the layers being thus shortened and the strain reduced. 

Very large coils have these sections subdivided into 
thin disks, separated also by hard-rubber, through which 
the wire passes as above. The wire of different sec- 
tions may also vary slightly in diameter, the coarser wire 
being used for the end sections and the finer for the 
middle, on account of the greater potential difference at 
the ends and induction at the center already mentioned. 

Ruhittkorff's Commutator. — The commutator invented 
by Ruhmkorff is often used in connection with the coil, 
either to reverse or to interrupt the current. Its construc- 
tion will be understood from Fig. 44. A hard-rubber 
cylinder mounted on a base has two brass cheeks Fand 




Fig. 44. 

V' connected by the pins v and v' respectively with the 
^xes a and b^ which, as well as the cheeks, are insulated 



ELECTRO MAGNETISM. IOC 

from each other by the rubber. Two brass springs con- 
nected with the binding-posts B and C press against the 
cylinder, which is mounted on brass supports connected 
with the binding-posts A and D. The battery wires 
connect also with A and Z>, and the terminals of the 
primary coil with B and C. 

When the cylinder is in the position shown in the cut, 
with the springs pressing against the insulating rubber, 
no current can pass; but when turned so as to bring the 
cheek V in contact with the spring attached to B^ and 
V with that attached to C, the current flows from -\- P 
through Aav' V\ through the coil from C to ^, and 
thence through Vvb D to — N. But when the cylinder 
is reversed, so that V connects with C, and V with B^ 
then the current from ^ P \.o — N flows through the 
coil in reverse order, by way of Aav'V\ from B to 
C, and thence, as before, through Vv b D to — N. 

The Coil a Converter. — The coil is not a generator but 
a converter, transforming the energy derived from the 
battery, or any dynamic generator, by increasing the 
potential difference, or E. M. F.; which must be done at 
the expense of a corresponding reduction in the volume 
of current, since otherwise there would be an increase 
of electric energy without a corresponding expenditure 
of chemical or other energy, which would be impossible. 
For all the energy is derived from the generator, and a 
certain percentage expended in operating the interrupter 
and overcoming the resistance of the primary coil, so 
that even when the interrupter is operated by external 
po^ver there is still a loss from the resistance of the 
primary. 

This energy, as has been shown, first enters the pri- 
mary coil, which, from its low resistance, carries a large 
current, while its high coefficient of magnetic induction, 
derived from the core, multiplies the lines of force cut 



I06 DYNAMIC ELECTRICITY AND MAGNETISM. 

by the secondary coil. The secondary, from the ex- 
treme fineness of the wire, has great resistance, and 
hence carries a very small current, but creates a great 
E. M. F., or potential difference, which in a large coil 
may equal many thousand volts, the convolutions being 
very numerous and each adding its quota to the coeffi- 
cient of mutual induction. 

But since, with wire of any given cross-section, the 
resistance increases directly as the length, and since, as 
shown, the E. M. F. also increases as the length and 
hence in the same ratio, the current must remain con- 
stant. But since, with a given size of coil, any variation 
in cross-section of wire produces a corresponding oppo- 
site variation in its length, from which must result a 
corresponding variation in the relative proportions of 
E. M. F. and current, it is evident that any resulting 
increase of current must produce a decrease of E. M. F., 
and any decrease of current an increase of E. M. F. 

The length of the spark, or discharge, depends both 
on the E. M. F., or electric pressure, and on the cross- 
section of the perforation made through air or other in- 
sulating medium ; for the length and cross-section of 
the perforation measure the resistance overcome, and 
any variation in either dimension must be compensated 
by an opposite variation in the other, otherwise there 
would result an increase of work without a correspond- 
ing increase of energy. Hence the great spark-length 
obtained by the coil is the result of the transformation, 
which, by reduction in volume of current, concentrates 
the electric energy on a fine line and impels it with a 
corresponding increase of E. M. F. The great advan- 
tage of the coil in this respect is shown by the fact that 
the longest spark obtainable without a coil from 1080 
silver chloride cells, the largest battery ever con- 
structed, is only ^\-^ of an inch, while Spottiswoode's 



eLe c tr oma gne tism. 1 07 

great coil gives, with 30 Grove cells, a spark of 42J 
inches in length. 

Electric Perforation. — Perforation, as used above, re- 
fers to the path by which the electric energy passes 
through a substance, using its material as the medium 
of transfer; displacement of this material being often an 
accompaniment of the discharge, though not a necessary 
consequence, since energy and not matter is thus trans- 
ferred. Paper, for instance, when thus perforated is 
displaced, while glass is pulverized on the line of dis- 
charge, with surrounding fracture and little or no dis- 
placement. A discharge through any insulating medium 
is termed disruptive. 

Physiological Effects of Faradic Current. — The faradic, 
or alternating, current of the coil, when passed through 
any part of a living body, produces a tingling sensation 
accompanied with muscular contraction, which may be 
mild or painfully severe, according to the strength of 
the current, as regulated in the manner already de- 
scribed. This current is now extensively employed in 
medical practice, and its use constitutes an important 
branch of electro-medical treatment. 

In ordinary lecture-room experiments it is received 
through metal handles, connected with the coil termi- 
nals, which may be held in the hands or otherwise ap- 
plied; but, for medical use, special electrodes, such as 
sponges and rollers having insulating handles, are used, 
by which the current can be applied by the physician or 
attendant as required. 

Discharge in Air and in Vacuo. — The intensely brilliant 
spark produced by the electric discharge in air at the 
ordinary density is due to the heat generated by the 
electric energy in this high resisting medium, which is 
rendered incandescent on the line of discharge by the 
intensely rapid vibration of its molecules. Hence the 



I08 DYNAMIC ELECTRICITY AND MAGNETISM. 

electric spark is a line, or fine cylinder, of incandescent air, 
often bent, contorted, or subdivided, whose molecules are in a 
state of intensely rapid vibration. 

The longest spark in air at the ordinary density is 
comparatively short, the energy being soon expended in 
overcoming the high resistance; but when the density 
is reduced by the production of a partial vacuum, the 
length of the discharge is proportionally increased. 
This may be done by a partial exhaustion of the air 
from a glass tube with the common air-pump, but is 
accomplished more effectively in the hermetically sealed 
vacuum tubes of Geissler, in which the density is re- 
duced by the mercur}'' pump to y^Vo ^^ ^'^ atmosphere, 
and platinum terminals sealed into the extremities. A 
discharge several feet in length may be obtained in such 
a tube with a small coil; the low resistance also per- 
mitting increase of cross-section, with change of color 
to the light pink seen in the aurora, which is a similarly 
diffused discharge. This change of color is a necessary 
consequence of the diffusion, since enlargement in the 
space occupied by the discharge produces a correspond- 
ing diffusion of the light and heat produced at each 
point and hence a proportional reduction of their in- 
tensity. 

Since the space occupied by the discharge increases 
in the same ratio as the reduction of the atmospheric 
density, a reduction to -^-^-^-^ of an atmosphere would 
give, with a tube of sufficient size, an enlargement of 
333^ times the space occupied by the same discharge in 
air at the ordinary density. 

But when the density is reduced to yo'o'o'TroT ^^ ^"^ 
atmosphere, as in Crooke's vacuum tubes, the medium 
becomes insufficient to carry the current with the same 
facility as in the lower vacuum, and we have resistance 
in the opposite sense to that found in air at the ordinary 



ELECTROMAGNETISM. IO9 

density, with many interesting phenomena described in 
"Elements of Static Electricity," in which this subject, 
and also the auroral discharge, is more fully discussed. 

Electric Gas Lighting. — The coil and battery are ex- 
tensively used for lighting the gas in churches and audi- 
ence halls where the burners are not easily accessible. 
For this purpose wire, properly insulated, is connected 
with the chandeliers, and interrupted at each burner so 
as to furnish short sparks which pass in series through 
the escaping gas and light it. 

Spark Coil. — The spark coil, as it is termed, is best 
adapted to this use; it consists of the primary coil and 
core, giving a short thick spark with strong current; 
the secondary coil and interrupter being dispensed 
with, reducing the resistance, risk of burning out, and 
expense. 

The term spark coil is also applied to the complete 
induction coil, when constructed for this or any similar 
purpose, where the main object is the spark rather than 
the current. 

The induction or influence machine — Holtz, Topler, 
or Wimhurst — is also used for the same purpose, as de- 
scribed in " Elements of Static Electricity." 



no DYNAMIC ELECTRICITY AND MAGNETISM, 



CHAPTER VI. 
ELECTRIC MEASUREMENT. 

Electric Measurement pertains to measurement of 
the force exerted in any way by electric energy, or of 
the resistance which opposes it, or of certain effects re- 
sulting from the mutual relations of this energy and 
resistance. It is dependent on certain physical con- 
ditions which will now be considered in their order. 

Electric Potential. — Potential in the physical sense is 
that condition of matter by virtue of which it is capable 
of exerting physical force. Thus we estimate the heat 
potential of a body by the effect it can produce on 
temperature; its gravity potential by the attractive 
force it can exert as a mass; its magnetic potential by 
the magnetic force it can exert; and its electric poten- 
tial by the electric force it can exert. 

Electric potential is designated relatively as positive, 
negative, or zero. Matter has positive electric potential, 
or is positively electrified, when its electric condition 
is higher than that of other matter to which it may be 
related either by contiguity of position or electric con- 
nection, so that it is capable of imparting electricity to 
it; it has negative electric potential when its electric 
condition is lower, so that it is capable of receiving™ 
electricity from such other matter; and zero electric 
potential when its electric condition is the same as that 
of the other matter, so that it can neither impart nor 
receive: and any variation in either condition must of 
pourse change these relative electric conditions. Hence 



ELECTRIC MEASUREMENT. Ill 

a body may have positive potential with reference to 
one of lower potential and, at the same time, nega- 
tive with reference to one of higher potential, or zero 
with reference to one of the same potential. 

Potential difference is conveniently represented by the 
symbol/, d. 

Electromotive Force. — Electromotive force has been 
already briefly referred to as '' that which moves or 
tends to move electricity from one point to another," 
and as being represented by difference of electric poten- 
tial. Hence it is the relative condition of electric force 
between different bodies or parts of a body; the tend- 
ency of electricity being always to move from higher 
to lower potential in the same sense as heat tends to 
move from higher to lower temperature. It is some- 
times represented as electric pressure^ in the same sense 
as water pressure in a reservoir, or steam pressure in a 
boiler, and the analogy is correct so far as t\\Q pressure 
is concerned; but water or steam pressure tends to 
move matter, while electric pressure tends to move 
molecular force, using matter as its medium. 

It is independent of the quantity of electricity gener- 
ated, and depends solely on potential difference, just as 
force in each infinitesimal drop of water in Niagara 
Falls is derived from the height of the falls and not 
from association with other drops. Hence small electric 
quantity may be combined with large electromotive 
force, or the reverse. The number of battery cells 
joined in parallel may be multiplied indefinitely, while 
the electromotive force, as has been shown, is only that 
of a single cell, each cell being, in this respect, indepen- 
dent of the others; and the same is true of the generat- 
ing parts of any other electric generator, when joined 
in parallel, as the parallel pairs of plates in a Topler 
machine, or the parallel coils of a dynamo. But when 



112 D YNAMIC ELECTRICI TV AND MA GNE TISM. 

these parts are joined in series, the electromotive force 
of each being added to that of the others varies as the 
number of such parts and as the value of this quantity 
in each. 

The symbol of electromotive force is E. M. F., as 
already given, but in mathematical formulae E alone is 
used. 

Electric Resistance. — Electric resistance, as briefly de- 
fined in Chapter I, is that which opposes electric move- \ 
ment, and may consist specifically in the molecular! 
constitution of the conductor or insulator; in counter- 
electromotive force, or counter-induction; in useful 
work; or in an artificial obstruction placed in the circuit 
for a useful purpose. 

In conductors it varies directly as the length of the 
conductor and inversely as its cross ssction, and also 
inversely as its conductivity; and as insulators must be 
regarded as inferior conductors, the same rule applies 
to them. But the relative electric resistance of the 
substance displaced by the insulator must also be con- 
sidered, since an insulator, as glass or vulcanite, required 
in construction, may displace dry air which has much 
higher electric resistance than either, and the insulation 
be thereby reduced. In such case resistance is in- 
creased by reduction in the cross-section of the insulator. 
But if a substance of lower resistance is displaced, in-', 
crease in cross-section of the insulator increases the 
electric resistance. 

In its effect, electric resistance in a conductor is similar 
to the frictional resistance produced in a pipe conveying 
a fluid, by an accumulation of loose material, such as 
moss or cotton waste, which obstructs the flow; but in 
its nature it is very different, fluid matter being trans- 
mitted by the pipe, but electric energy by the conductor. 

Insulation and Conductivity. — Resistance is the oppo- 



ELECTRIC MEASUREMENT. II3 

site of conductivity, and very high resistance, when 
applied to a certain class of bodies, is termed insulation* 
Conductivity is that quality of a body which facilitates 
electric transmission, while resistance or insulation ob- 
structs it. Each varies inversely as the other, but there 
is no well-defined boundary between them; every con- 
ductor having a certain amount of resistance, and every 
insulator a certain amount of conductivity. Where 
conductivity is found to predominate, as in the metals, 
the term conductor is applied, and where resistance pre- 
dominates, as in glass and vulcanite, the term insulator 
is applied. Silver and copper are metals of the highest 
conductivity and consequently of the lowest resistance. 
German-silver and bismuth have high resistance and 
hence low conductivity. Glass and vulcanite have high 
insulation and correspondingly low conductivity, so low 
that the term conductor is never applied to them, nor 
is the term insulator ever applied to silver or copper. 
Hence a conductor is any substance of such low resist- 
ance that it can be used practically for the transmission 
of electricity, and a non-conductor or insulator is any 
substance of such high resistance that it can be used 
practically to prevent such transmission. 

If electricity is a mode of molecular motion by which 
energy manifests itself, difference of molecular constitu- 
tion in different bodies would easily account for these 
varied results. Such difference might consist in varia- 
tion in the size, shape, or relative arrangement of the 
molecules, or in a combination of such causes. Molecu- 
lar arrangement in a conductor might be such as to 
produce harmony of movement by which undulations 
would be rapidly propagated, while in an insulator a 
different arrangement might produce conflicting move- 
ments, by which they would neutralize each other and 



114 DYNAMIC ELECTRICITY AND MAGNETISM. 

thus prevent transmission, as already explained in regard 
to magnetism in Chapter IV. 

Electric Current— Current, as stated in Chapter I, is 
that electric condition in a conductor which results from 
electromotive force modified by resistance; and its 
mathematical quantity is ascertained by dividing the 
former by the latter. It pertains exclusively to what is 
understood as electric movement, and is used in the same 
sense when applied to this movement in a conductor, as 
the same term when applied to the flow of water, steam, 
gas, or any other fluid, in a pipe; and in this sense also 
are used the terms current intensity, quantity, volume, 
strength and resistance; and on this principle all the 
various kinds of electric apparatus pertaining to current 
are constructed, and current estimates and measure- 
ments made. 

In the present imperfect state of electric knowledge 
this conventional form of expression is convenient and 
admissible, provided the distinction between an electric 
current and a fluid current is kept strictly in mind, the 
former being a flow of energy, the latter a flow of mat- 
ter. Our actual knowledge of the nature of an electric 
current is very limited; the generator creates E. M. F. 
at one end of the conductor, and a molecular movement 
is supposed to take place by which electric energy is 
instantly transmitted. Theoretically this movement is 
in the form of transverse vibrations, but as a matter of 
fact its nature is unknown; the only well established 
fact concerning it being that there is no transmission of 
a fluid, as was formerly supposed, or of other matter, 
energy alone being transmitted, using matter as its 
medium; and it is the effect produced by the energy on 
this medium, which is known as the electric current, or 
electricity in process of transmission. 

Ohm's Law. — The law by which the strength of an 



ELECTRIC MEASUREMENT. II5 

electric current is determined was discovered by the 
German electrician, Ohm, and is briefly as follows : 

The strength of an elect^-ic current varies directly as the 
electro7notive force by which the current is impelled^ and 
inversely as the total resistance encountered. 

From this law are derived the following formulae by 
which either of the three factors represented by the 
symbols C, E, R can be found when the other two are 
known : 

Formula for finding current, C = -^. 
Formula for finding E. M. F., E = CR. 
Formula for finding resistance, R = —. 

Electric Units. — In order to render possible the calcu- 
lations required in estimating electromotive force, resist- 
ance, and current, certain units of measurement are 
required, some of which have already been briefly de- 
fined in connection with batteries. They are appropri- 
ately named after different distinguished electricians. 

The International Electric Congress which met at 
Paris in 1881, and again in 1884, revised these units, 
giving them a definite value referable to fixed standards, 
and the units thus established are distinguished as 
'legaV and accepted as authoritative. 

The C. G. S. mechanical unit is taken as the basis of 
the units by which electric force may be represented in 
absolute measure. The initial letters, C. G. S., are the 
symbols of the three factors, space, mass, and time ; C. 
standing for centimeter, G. for gramme, and S. for 
second ; hence this C. G. S. unit represents the work 
accomplished by the movement of a mass equal to one 
gramme, through a space equal to one centimeter, in one 
second, and is known as the ei-g. 



Il6 DYNAMIC ELECTRICITY AND MAGNETISM. 

The Volt. — The volt is the unit of electromotive force, 
and was formerly represented by the E. M. F. of a 
battery-cell nearly equal to that of the Daniell ; but as 
this is a variable quantity, a definite value was given 
this unit by the adoption of a quantity represented by 
100,000,000 C. G. S. units as its equivalent, which is 
therefore the amount of electric energy which, if con- 
verted without loss, would equal this amount of mechani- 
cal force. But as such a large number is inconvenient 
to write, the equivalent expression, 10*, has been adopted 
in its stead, and the same method of abbreviation fol- 
lowed in representing the other electric units. Hence 
the legal volt equals 10* C. G. S. units of E. M. F. ; but 
in approximate estimates, the E. M. F. of a Daniell cell, 
which is about 1.05 volts, is usuall}^ sufficiently ac- 
curate. The microvolt equals yttoo-oTo" ^^ ^ volt. 

The Ohm. — The unit of electric resistance is the ohm. 
It was formerly represented by the resistance of a given 
number of feet of wire of a given gauge ; a very unre- 
liable standard, requiring a different length for each dif- 
ferent metal, and subject to great variation from differ- 
ence of quality or temperature, or slight difference of 
gauge. 

The standard resistance adopted by the Electric Con- 
gress is that of a column of pure mercury, 106 centims. 
in length, and i sq. millim. in cross-section, at the tem- 
perature of 0° C; which is nearly equal to 10^ C. G. S. 
units ; the resistance of a similar column, 106.21 centims. 
in length, being the exact equivalent, but to avoid the 
fraction the standard was fixed as above. Hence the 
legal ohm equals 10^ C. G. S. units of resistance. The 
7negohm equals a million ohms. 

The Ampere. — The unit of current strength, or volume, 
is the ampere, and is derived from the two preceaiiig 
units in accordance with Ohm's law, by dividing the 



ELECTRIC MEASUREMENT. WJ 

unit of E. M. F. by the unit of resistance. Hence, since 

C—-^, I amp. = — g = lo = — . 
R lo^ lo 

Hence the legal ampere represents current strength 
equal to ^^g- of a C. G. S. unit. It does not include time 
as an element, but refers exclusively to the strength of 
current flowing in a conductor at any instant, as repre- 
sented in cross-section at any point. The milli-ampere is 
the thousandth part of an ampere. 

The Ampere-Hour. — The ampere-hour is a unit derived 
from the last, in which the element of time is included. 
It represents a current of one ampere flowing through a 
conductor for one hour, or its equivalent in a greater 
current for a less time or a less current for a greater 
time, as two amperes for half an hour, or half an ampere 
for two hours. It is of recent origin, but is sanctioned 
by general use, and is often convenient in electric calcu- 
altions. 

The Coulomb. — The unit of current quantity with 
reference to time is the coulomb. It is derived from the 
ampere, and represents the quantity of electricity which 
flows for one second with a current strength of one 
ampere. Hence any variation either in the time or 
strength of a current produces a corresponding varia- 
tion in the quantity represented in coulombs, while if 
one factor varies inversely as the other the quantity re- 
mains constant; a ten-ampere current flowing for one 
second or a one-ampere current flowing for ten seconds 
represents ten coulombs. And since there are 3600 
seconds in an hour, 3600 coulombs equal one ampere- 
hour. The legal coulomb, being derived from the 
ampere, equals io~^, or -^-^^ of a C. G. S. unit of current 
quantity. 



Il8 DYNAMIC ELECTRICITY AND MAGNETISM. 

The Farad. — The electric unit of capacity is X\\q. farad. 
It represents the storage of one coulomb of electricity in 
a condenser; and when such storage raises the potential 
to one volt, the capacity equals one farad. The legal 
farad equals lo"^, or t-qo-o-dVoooo? of a C. G. S. unit of 
capacity. 

The Microfarad. — The farad being inconveniently large 
for practical use in estimating the capacity of condensers, 
the microfarad, representing one millionth of a farad, has 
been adopted in its stead. Hence the microfarad equals 
Io~'^ or iooooooo^ooooooo o> of a C. G. S. unit of capacity. 

The Watt. — The unit of electric power is the watt^ 
named after the inventor Watt. It is derived from 
E. M. F. and current combined, neither of which taken 
alone is a correct representative of electric power ; E. 
M. F. representing pressure, while current represents 
pressure modified by resistance ; hence there might be 
large E. M. F. with small power, or the reverse, in pro- 
portion to the relative resistance ; or current might 
remain constant while power varied. Hence, to obtain 
an accurate expression for electric power, or rate of 
work, the E. M. F. is multiplied into the current, — that 
is, the volt into the ampere. The legal watt then equals 
one volt multiplied into one ampere, the product being 
lo' C. G. S. units of power, — lo' X io~^=io'. The 
term volt-ampere is synonymous with watt. 

The Electric Horse-Power. — The electric horse-power, 
which is the equivalent of the mechanical horse-power, is 
represented by 746 watts, equal to 

746 X 10^ = 7,460,000,000 C. G. S. units of power. 

Different Kinds of Electric Measurement. — The electric 
measurement here considered pertains to dynamic elec- 
tricity ; and since much of the apparatus by which elec- 
tricity in this form is generated, and by which it is 



ELECTRIC MEASUREMENT. IIQ 

measured, is constructed with reference to the reciprocal 
relations between electricity and magnetism, the units 
are usually termed electromagnetic to distinguish them 
from electrostatic units, which represent electric force 
alone, and from magnetic units, which represent magnetic 
force alone. 

Instruments for electric measurement are constructed 
either on the principles of electric attraction and repul- 
sion, on the relations between electricity and magnet- 
ism, on the heat developed by the electric current, or 
on the amount of metal deposited or gas generated by 
electrolysis. 

Electrometers.— The instruments by which electro- 
static force is measured are known as electrometers^ and 
measure either the absolute force by which one electri- 
fied body attracts another by direct movement, as in the 
attracted-disk electrometer ; or the relative force by 
which one repels another by a rotary movement, as in 
the torsion balance; or the combined relative effects of 
attraction and repulsion by rotary movement, as in the 
quadrant electrometer. As all these electrostatic instru- 
ments and methods of measurement are fully described 
in the author's " Elements of Static Electricity," further 
reference to them here is unnecessary. 

Galvanometers. — Instruments for electric measure- 
ment constructed on the principle that the magnetic 
needle tends to assume a position at right angles to that 
of the electric current were formerly known exclusively 
as galvanometers^ a term still applied to the older instru- 
ments of this class, while certain improved instruments 
recently constructed on this principle are known as volt- 
meters and ammeters, the former used to measure elec- 
motive force, and the latter current strength. 

Instruments indicating the presence and direction of 
electric currents, as the galvanoscope, Schweigger mul- 



120 DYNAMIC ELECTRICITY AND MAGNETISM. 

tiplier, and astatic needle, have already been described in 
Chapter IV, but none of these measure current strength, 
though roughly indicating its amount, while the gal- 
vanometer, constructed on the same principles, is a much 
more accurate instrument. Its general construction 
consists of a magnetized needle, poised so as to have a 
free horizontal rotary movement, and inclosed within a 
coil of insulated copper wire through which the electric 
current can flow; the strength of the current being 
measured by the needle's deflection as shown on a 
graduated circle of 360°. 

Galvanometers are adapted only to the measurement 
of direct currents, and are but slightly affected by 
alternating currents. 

In every galvanometer except the astatic, the needle 
and the vertical plane of the inclosing coil are set in the 
plane of the magnetic meridian, so that the deflecting 
force of the current acts at right angles to the horizontal 
component of the earth's magnetism, the former tending 
to rotate the needle into a position at right angles to the 
direction of the latter. Hence it is evident that the 
amount of the deflection never can exceed 90°, since at 
this angle the position of the needle is normal to the 
direction of the current, and the force represented by 
the angle at a maximum. 

But the deflecting force does not vary in the same 
ratio as the angle of deflection, since the needle receives 
the full effect of this force only when in the vertical 
plane of the coil, which in this case coincides with that 
of the magnetic meridian, while in every other position 
only a portion of this force acts on it, and the strength 
of this effective portion varies inversely as the angle of 
deflection. 

This matter will be better understood, especially by 



ELECTRIC MEASUREMENT. 



121 



those not familiar with the measurement of angles, Dy 
reference to Fig. 45. 

Let the line NS represent the needle in the plane of 
the magnetic meridian, poised at its center C, so that it 
can be rotated by the deflecting force into the position 
WE^ or any intermediate position ; the force acting on 
its north pole, N, tending to rotate it toward E, and that 
acting on its south pole, 5, tending to rotate it toward 
W. When the needle has thus been turned from the 
position -A^^S", the deflecting force acts on it obliquely, its 




Fig. 45. 

effective component on the north pole, when in the 
position AR^ being represented by the line /C, while 
the remainder acts along the needle's length, and is not 
represented by the angle of deflection ; a similar result 
being true of the deflective force on the south pole ; 
hence the effective part of this force in the position AR 
is to that in the position NS^ as IC to NC ; and so when 
the north pole has been deflected to /?, Z>, F, or H, the 
effective part, as compared with that represented by 



122 DYNAMIC ELECTRICITY AND MAGNETISM. 

NCf is represented respectively by the lines /C, KC, LCy 
and MC. 

But the effective part represented by each of these 
lines belongs to a current of increased strength, other- 
wise it could not produce the increased deflection, and 
hence, though representing a constantly decreasing incre- 
ment of the total force, its actual strength is increasing 
directly as the angle of deflection ; so that the effec- 
tive part, represented by the short line MC, is as much 
stronger than the entire deflective force represented by 
NC as the angle NCH is greater than zero. 

Measurement of Angles. — Angles are measured by cer- 
tain functions known as sines, cosines, and tangents. 
Take any angle, as NCA^ and with any part, NC, of one 
of the inclosing lines, as radius, and the point C, where 
the lines meet, as center, describe a circle ; and from the 
point A, where the other inclosing line meets or inter- 
sects the circumference, draw a line, AI, perpendicular 
to NC\ the ratio between the length of this line and 
radius is the sine of the angle. And the length of radius 
being taken as the unit, the sine is represented by the 
length of this perpendicular, which therefore is the 
measure of the angle. Hence each of the lines, BJ, 
DK, FL, HM, perpendicular to NC, is, like AI, the 
measure of the angle which it subtends. The length of 
this perpendicular may vary from radius to zero, but 
evidently can never exceed radius. 

The cosine is the ratio between the length of radius 
and that part of it included between the center and the 
point where the perpendicular representing the sine 
meets it. Hence, radius being unity, CI represents the 
cosine of the angle NCA, and may also be taken as its 
measure ; the value of the cosine varying inversely as 
that of the sine. In like manner C/, CK, CL, and CM 



ELECTRIC MEASUREMENT. 1 23 

represent the cosines of the other angles mentioned, and 
hence measure them. 

The tangent is a straight line which touches the cir- 
cumference of a circle, or arc at any point, but which, 
if produced, does not cut it, as NT ; and hence it forms 
a right angle with radius at the point of contact. For 
any angle less than a right angle, it is included between 
the line coinciding with radius at the point of contact 
and a straight line drawn from the center and produced 
to meet it, and hence it subtends the angle formed by 
these lines. Thus ^^5 is the tangent of the angle NC^^ 
and each of the lines, iV^io, N 1^, 7V^20, and N 2^^ the 
tangent of the angle which it subtends. 

The length of the tangent varies from zero to infinity; 
the tangent of a right angle being infinite, since it is 
perpendicular to one of the inclosing lines and parallel 
to the other, and hence can never meet the latter. 

Angular Measurement of Deflective Force. — It has been 
shown in Chapter IV that the horizontal force of the 
earth's magnetism, by which the needle is deflected, 
must vary as the tangent of the angle of deflection; but 
in the galvanometer this force is represented by that of 
the current flowing in the coil, hence the same rule ap- 
plies ; so that if the tangent be laid off into equal spaces, 
as in the figure, and lines from the dividing points be 
drawn to the center, those spaces must represent equal 
increments of current strength, though the increments 
of the circumference included between these lines and 
also the cosine, which represents the effective component 
of the deflective force, constantly decrease as the angle 
of deflection increases. Hence the total deflective force, 
representing the current's strength, does not vary as the 
arc through which the needle rotates, but as the tangent 
of the including angle. 

For instance, the ratio of strength between a current 



124 DYNAMIC ELECTRICITY AND MAGNETISM. 

producing a deflection of io° and one producing a de- 
flection of 20° is not that of 10° to 20°, but of tan 10° 
to tan 20° ; for it requires a current of much more than 
double the strength to double the arc, since, as already 
shown, only that portion of the total force represented 
by the cosine is effective in producing the deflection; 
but the tangent of 20" is much more than twice the 
length of the tangent of 10°, and represents the total 
increment of force, effective and non-effective, while the 
cosine represents only the effective portion. Now since 
it has been shown that the strength of the effective in- 
crement varies as the angle, it is correctly represented 
by the angle's sine. 

Hence the effective deflective force varies as the 
COSINE, its STRENGTH as the SINE, and the total strength 
OF current as the tangent of the angle of deflection. 

Calibration of Galvanometer. — A galvanometer may be 
calibrated by ascertaining from comparison with a simi- 
lar standard instrument, or otherwise, the different 
degrees of current strength represented by different 
degrees of deflection ; and these results being tabulated 
are a correct guide for the use of the instrument so long 
as the magnetic strength of the needle remains unim- 
paired, and the functions of other parts affecting the 
deflection remain constant. 

Melloni used the differential deflections of opposite 
electric currents produced by heat as a means of cali- 
bration ; and the term seems, perhaps for this reason, 
to have been derived from thermometric calibration, to 
which it is analogous. 

All instruments for electric measurement require jew- 
elled bearings for the rotating parts, to reduce the fric- 
tion to the minimum. 

Sine Galvanometer. — The deflective force may be meas- 
ured either by the sine or the tangent, according to the 



ELECTRIC MEASUREMENT, 1 25 

construction of the galvanometer. Where great sensi- 
tiveness is required the sine galvanometer is preferred. 
Its essential features are a long needle and an inclosing 
coil of only sufficient diameter to permit the needle's 
free oscillation, and which can be rotated horizontally. 
Its construction will be understood from Fig. 46. 
The needle is mounted at the centre of a vertical coil, 



Fig. 46. 

composed of a number of convolutions of insulated cop- 
per wire wound on a circular grooved brass frame^ and 
underneath the coil is mounted a circle, graduated to 
correspond to a similar one in proximity to the needle 
above; the centre of each being in a vertical line with 
the centre of the needle. The upper circle is attached 
to the frame of the coil, so that both can be moved 



126 DYNAMIC ELECTRICITY AND MAGNETISM. 

horizontally by an index lever, shown below, through 
any number of degrees indicated on the lower circle by 
the index. 

The instrument being set with the plane of the coil in 
the magnetic meridian, parallel to the needle, which 
points to zero, and a deflection being produced by the 
passage of ihe current to be measured, the needle rotates 
out of the plane of the coil to a position where the mag- 
netic field is weaker ; the coil is then turned in the same 
direction as the needle, its approach producing further 
deflection, till its plane again coincides with the needle, 
which again points to zero. In this position the deflec- 
tive force of the current is evidently just equal to the 
opposing horizontal force of the earth's magnetism, 
which would bring the needle back to its original posi- 
tion if the deflective force were withdrawn. The num- 
ber of degrees through which the coil has been turned 
being noted on the lower scale, the sine of the corre- 
sponding angle indicates the current strength, to which 
it bears a certain definitely varying ratio, as has been 
already shown. 

This mode of measurement is approximately accurate 
for angles of less than 20°, in which the values of the 
sine and tangent are nearly equal, but is not reliable for 
larger angles, the difference in those values being too 
great, as has been shown. 

Tangent Galvanometer. — The tangent galvanometer, 
though less sensitive than the sine galvanometer, is sim- 
pler in construction and more accurate for large deflec- 
tions by strong currents, and hence is generally pre- 
ferred. Its essential features are a short needle, and a 
coil of relatively large diameter, varying from ten inches 
to one meter. The needle is usually about three quar- 
ters of an inch in length, diamond-shaped, with an 
aluminium pointer of convenient practical length attached 
at right angles to its polar diameter. The large diam- 



RLE C TRIC ME A S U REM EN T. 



127 



eter and circular form of the coil are to create a field of 
approximately uniform strength within the small central 
area in which the needle rotates ; the inner lines of 
force from the current converging to the centre ; and 
the needle is made short so as to be confined as closely 
as possible to thi^ small central space, where the field 
is most uniform. 

Fig. 47 shows an instrument of this class ; its coil 
consisting of a single turn of copper wire, having prac- 




tically no resistance, and not requiring a supporting 
frame. The coil terminals are connected with two 
tubes shown at the base, one inside the other, insulated 
from each other and furnished with binding-screws. 
The needle-case is from four to five inches in diameter. 
The tangent values are sometimes laid off on the 
scale of the instrument in the manner shown in Fig. 45 ; 



128 DYNAMIC ELECTRICITY AND MAGNETISM. 

hut as it is difficult to do this with requisite accuracy, 
the scale of degrees is usually preferred, the values of 
the tangents corresponding to the deflections being 
easily ascertained from a table. 

The Helmholtz-Gaugain tangent galvanometer is 
illustrated by Fig. 48. The needle is placed at the cen- 
tre of a straight line connecting the centres of two 
separate coils of equal size, set parallel to each other at 
a distance apart equal to their radius. This arrange- 




FlG. 48. 

ment insures much greater uniformity of field than can 
be obtained from a single coil, and still greater uni- 
formity could be obtained by the addition of a third 
coil, midway between the two, and of such diameter 
that each of the three should be equally distant from 
the centre of the needle. The two-coil method was pro- 
posed by Helmholtz, while Gaugain proposed placinp; 



ELE C TRIG M EA S U RE MEN T. 



129 



the needle in the same relative position on one side of a 
single coil. 

The instrument here shown has two sets of coils, 
marked A and B, four in all, connected with binding- 
screws at the base of each circular support, as shown, 
one coil of each set on each support. The A set has 
very low resistance, only a small fraction of an ohm to 
each coil ; each being composed of about four turns of 
No. 12 copper wire. The B set has very high resistance, 
10 to 12 ohms to each coil, each composed of No. 26-30 
wire. The needle is suspended by an untwisted silk 
fibre inclosed in a vertical tube, and adjusted by the 
screws shown at top, so that it rotates without friction 
and against only slight torsion. 

Astatic Galvanometer. — A very sensitive galvanometer, 
originally invented by Nobili, may be constructed with 
the astatic needle described and illustrated on page 
73 ; which, being approximately independent of the 
earth's magnetism, is deflected by a very slight current. 
Fig. 49 shows the construction. 

Two short needles with poles reversed are attached 
to a common support, which also carries a light pointer 
of convenient length. The coil is 
flat and usually of sufficient hori- 
zontal diameter to inclose the lower 
needle entirely in any position; while 
the upper needle rotates over its 
upper surface, and the pointer over 
a dial-plate with scale above. 

The needle is suspended at the 
centre of the coil from a vertical sup- 
port by a single fibre of silk, or by 
two parallel fibres hung near each 
other; the latter method being known 
as bi filar susiyension, its obiect being to bring the needle 




130 DYNAMIC ELECTRICITY AND MAGNETISM. 

to rest in a fixed position more perfectly than can be 
done by the torsion of a single fibre; the needle being 
raised slightly when, by its deflection, the two threads 
are twisted out of parallelism, and its weight tending to 
bring them back to the parallel position. The suspen- 
sion is adjusted by the thumb-screw shown above; the 
needle being set parallel to the vertical plane of the 
coil; and as it is impossible to make a needle perfectly 
astatic, both should also be parallel to the plane of the 
magnetic meridian. A glass shade affords protection 
from air-currents. 

The readings, for the reasons already given, are only 
approximately accurate, and for deflections greater 
than 20° unreliable; but the instrument can be cali- 
brated for larger deflections. 

Thomson's Reflecting Galvanometer. — This instrument, 
invented by Sir William Thomson for telegraphing 
through long submarine cables, is exceedingly sensitive. 
Its construction is shown by Fig. 50; its principle being 
practically that of a tangent galvanometer with a long 
pointer and tangent scale. 

At the center of a line connecting a pair of small coils 
of equal size and resistance, is suspended, by a silk fibre, 
a diminutive concave mirror, of about i centim. diameter, 
with a little needle, made usually of a piece of watch- 
spring, attached to its back; the weight of both not ex- 
ceeding one or two grains. A small circular opening 
in the case, directly opposite the mirror, widening out- 
ward, admits the light from a lamp connected with the 
graduated scale shown in Fig. 51. 

This scale is placed in front of the galvanometer, at a 
distance of about 36 inches, and the lamp is placed in 
the box at the right which excludes the direct rays. 
The light is transmitted through a tube terminating in 
a small circular opening, from which a beam falls on 



ELECTRIC MEASUREMENT. 



31 



the small mirror shown just below the centre of the 
scale, and is reflected to the galvanometer mirror, and 
thence back to the scale; the mirror adjustments being 
such that a small spot of light, concentrated by a lens 
in the galvanometer, shown in the cut, is reflected on 




Fig. 50. 

zero of the scale, when no current is passing, and moved 
to the right or left, according to the direction of the 
current, to a distance corresponding to the current 
strength. The shadow of a fine wire, stretched in front 



132 DYNAMIC ELMCTRiCITY AMD MAGNETISM. 

of the galvanometer mirror, indicates the exact centre 
of the spot of light, which is adjusted to zero by a curved 
magnet, attached above to a vertical rod, with its poles 
in opposition to those of the needle, and which can be 
moved to any required position vertical or horizon- 
tal. 

The pointer being the ray of light, 36 inches long, the 
slightest deflection is prominently indicated on the 
scale ; a current produced by dipping the points of a 




Fig. 51. 

brass pin and a sewing-needle into a drop of salt water, 
moving the spot of light half the length of the scale. 

The coils can be removed and coils of any required 
resistance up to 5000 ohms substituted ; and as these 
and similar delicate measuring instruments are liable to 
injury from powerful currents, which also produce de- 
flections too great for accurate measurement, shunts of 
fine wire are provided, separate from the instrument, by 
which fractions of the current, of measurable strength, 
are transmitted ; and the respective resistances of coil 
and shunt being known, the entire current strength can 
be ascertained. 



I 



ELECTRIC MEASUREMENT. 133 

The requisite light can be furnished by an ordinary- 
kerosene lamp, but that of an electric lamp or a lime 
burner, when obtainable, is far superior. 

Fig. 52 shows another style of the same instrument 




Fig. 52. 

with four coils in two sets, upper and lower, having any 
required resistance up to 8000 ohms. 

Differential Galvanometer. — This instrument is con- 
structed with two coils of equal size and resistance, be- 
tween which the needle is mounted at the central point, 
and through which currents may be transmitted simul- 
taneously in opposite directions and their relative 



134 DYNAMIC ELECTRICITY AND MAGNETISM. 

Strength compared : if equal, there is no deflection ; but 
if unequal, the relative difference in strength is shown 
by the amount of the deflection. 

Ballistic Galvanometer. — A ballistic galvanometer is 
one constructed with a needle weighted by inclosing it 
in lead or otherwise, so that the impulse given it by a 
transient current of too short duration to be measured 
in the ordinary way may be developed slowly by the 
needle's momentum, so that the amount of deflection 
can be more easily observed. 

When used to measure current quantity, as indicated 
by current strength in the discharge of a condenser, the 
sine of half the angle of deflection produced by the first 
swing of the needle is taken as proportional to the 
quantity of the transient current thus produced. 

Common Galvanometers. — Galvanometers of various 
styles and sizes are constructed for ordinary practical 
use, usually with flat coils of various degrees of resist- 
ance. Such instruments are often better adapted to 
measurements where only approximate accuracy is 
required than those of finer construction, but are not 
suitable for strict scientific work. 

Voltmeters and Ammeters. — Galvanometers measure 
only current strength, usually in degrees of an arc, but 
it has become important in the progress of electric de- 
velopment to measure also electromotive force, and to 
express the measurements of both E. M. F. and current 
strength in volts and amperes, either directly or in 
terms easily reducible to those units : for this purpose 
voltmeters and ammeters are constructed. 

The difference between these two instruments con- 
sists chiefly in the respective resistance of each, and its 
relative position in use ; the voltmeter having high re- 
sistance and being placed in a derived circuit between 
the points whose difference of potential is to be meas- 



ELE C TRIG ME A S UREMENT. 



135 



ured, while the ammeter has low resistance and is placed 
directly in the main circuit at any point where current 
strength is to be measured. 

It will be noticed that an unmagnetized, soft-iron 
needle, or armature, is an important feature of many of 
these instruments. 

The Weston Voltmeter. — This instrument, shown by 
Fig. 53, incloses within its case a powerful steel horse- 




Fig. 53. 



shoe magnet, the poles of which project into the narrow 
space in front and are attached to two soft iron pole- 
pieces, as shown in Fig. 54. These inclose a circular 
space, within which is mounted a soft-iron armature 
core, maintained in a fixed central position by attach- 
ment to a brass yoke which connects the pole-pieces ; 
part of this yoke, with its right-hand connection and a 
central projection for attachment of the core, being 
shown. 

A light copper frame, f of an inch wide, and wound 
with a coil of fine, insulated, copper wire, surrounds the 
core, and has a limited rotary motion, on jewelled bear- 
ings, in the narrow space between the core and pole- 



136 DYNAMIC ELECTRICITY AND MAGNETISM. 

pieces, which is just wide enough to allow rotation with- 
out contact. 

The terminals of the coil are connected above and 
below with two flat springs, oppositely coiled, and so 
attached to the copper frame and adjoining parts as to 
maintain the coil in a fixed position, when the springs 
a'^ not under tension, and bring a light aluminium 




I 



Fig. 54. 



pointer, attached to the frame, to zero of the scale on 
the left. 

These springs are made of a special, non-magnetic 
alloy, and are placed in opposition to neutralize the 
effects of expansion and contraction under variations of 
temperature. 

A resistance coil, mounted within the case, makes 
electric connection, by one of its terminals, with one of 



ELECTRIC MEASUREMENT, 137 

the springs, while the other terminal is connected with 
the front binding-post on the left. Another connection 
with the rear binding-post on the same side taps this 
coil at a point nearer the spring, so as to include a much 
lower resistance. The other spring is connected with 
the binding-post on the right, back of which is a contact 
key and a calibrating coil. This part of the circuit can 
be closed permanently, after calibration, by depressing 
the key and giving it a quarter-turn. 

When connections with an electric source are made by 
the right binding-post and either of the two on the left, 
the current enters and leaves the copper coil through 
the springs, its direction and the winding being such as 
to produce deflection from left to right; the coil tending 
to rotate into a position at right angles to the lines of. 
magnetic force, in opposition to the tension of the 
springs. And the instrument being calibrated in ac- 
cordance with the resistance of its coils, the deflection 
of the pointer will indicate the difference of potential in 
volts; since with a given resistance the E. M. F., or po- 
tential difference, varies directly as the current strength. 

The entire resistance is to that of the sectional part 
in the ratio of 20 to i; the divisions of the scale being 
in volts for the outer reading, corresponding to the high 
resistance, and the same in twentieths of a volt for the 
inner reading, corresponding to the low resistance, as 
shown. Hence the E. M. F. which will produce a de- 
flection of one division, when connection is made with 
the front binding-post on the left, will produce a deflec- 
tion of twenty divisions when connection is made with 
the rear binding-post. 

The high-resistance circuit is used for apparatus gen- 
erating strong currents, as dynamos, and the low-resist- 
ance circuit for apparatus generating weaker currents, 
as primary batteries, on account of its greater sensitive- 



138 DYNAMIC ELECTRICITY AND MAGNETISM, 

ness: and as a dynamo current would be likely to injure 
or destroy the copper coil, if admitted through the low 
resistance, the rear post is protected from accidental 
contacts by an outer covering of hard -rubber. In some 
of the instruments all the posts are similarly protected; 
the rubber also preserving the contacts from oxidation. 
The scale readings also vary in different instruments. 

The deflection of a current-bearing coil in a magnetic 
field of special strength gives this instrument great 
superiority over instruments depending on the deflection 
of a steel or soft-iron needle; the magnetic action being 
stronger, and its relation to the current more direct. 
The constancy of the instrument is dependent solely on 
the constancy of the magnet, the springs, and the inter- 
nal resistance. 

The Weston Ammeter. — The construction of the Wes- 
ton ammeter is similar to that of the voltmeter, but 
simpler ; the chief differences being that the copper 
coil is of coarser wire, having much lower resistance, 
and the resistance coil is not required: hence there are 
only two binding-posts and a single circuit, directly 
through the copper coil and springs. 

The scales for different instruments range from 5 
amperes, with divisions of -}-^ oi an ampere, to 100 am- 
peres, with divisions of i ampere, according to the rela- 
tive resistance of the coils. 

The Weston Milliammeter. — This instrument has the 
same construction as the ammeter but lower resistance. 
Instruments of two different resistances, with scales of 
corresponding difference, are constructed; one of 300 
milliamperes, with scale divisions of 2 milliamperes 
each; and the other of 600 milliamperes, with scale 
divisions of 4 milliamperes each. 

A milliampere being yoVo^ ^^ ^^ ampere, it is evident 
that these instruments are capable of measuring very 



ELECTRIC MEASUREMENT. 



39 



low currents, especially as the scale divisions are read- 
able to fifths; so that the smaller instrument can indi- 
cate a current of -|- of 2 milliamperes, = -g-^Vo- ^^ ^^ 
ampere. 

The Wirt Voltmeter. — This instrument, illustrated by 
Fig. 55, is constructed on the principle of ascertaining 
the E. M. F. to be measured by comparison with a known 




E. M. F.; each being proportional to a resistance having 
similar conditions through which the measurement is 
made. 

The case incloses two Clark cells, each having a con- 
stant E. M. F. of 1.43 volts, the connections being so 
arranged that either can be employed alone, or the two 
joined in series so as to obtain an E. M. F. of 2.86 volts. 



I40 DYNAMIC ELECTRICITY AND MAGNETISM. 

Under the glass cover is shown a small galvanometer, 
with magnetic needle, light aluminium pointer, and 
terminal wires connected with the coil; also a small 
scale, not shown, under the pointer, having a limited 
range, in opposite directions, from o at the centre. 

Extending round the case inside is a coil of german- 
silver wire, having a resistance of about 2500 ohms, one 
terminal of which is attached to one of the binding-posts 
shown on the right, marked -\-, while a sliding contact, 
which can be moved to any required point on this coil, 
is connected with the other binding-post, marked — ; 
and this contact is attached to the rim of the hard- 
rubber cap, shown above, which can be rotated on the 
interior part of the cap, on which is shown a scale 
graduated in volts, from ij to 120. By rotating this 
rim, a short index, attached to it, is moved to any re- 
quired point on the scale, the sliding contact being 
moved simultaneously, so as to include any resistance 
required between the terminals of the binding-posts. 

The galvanometer circuit also includes a certain por- 
tion of this coil, having a known resistance calibrated 
with reference to the known E. M. F. of the battery 
cells, which are also included in this part of the circuit. 
A contact key, shown on the left, closes this circuit 
through the galvanometer, producing deflection of the 
needle and attached pointer. 

If connection with a generator whose E. M. F. is to 
be measured be made through the binding-posts, so 
that the current shall oppose the meter's battery cur- 
rent, the needle will be deflected, when the contact key 
is closed, so long as the generator current is stronger or 
weaker than that of the battery. 

Let the instrument be so placed that the earth's mag- 
netism shall bring the galvanometer pointer to o on the 
small scale ; and let the rim be turned so as to bring the 



ELECTRIC MEASUKEMENT. I4I 

Attached index near the probable E. M. F. on the large 
scale ; then, deflection being produced by closing the 
contact key, let the rim be turned so as to include suffi- 
cient resistance to equalize the opposing currents and 
bring the galvanometer pointer back to o ; the index 
will then show the E. M. F. of the generator in volts on 
the large scale. For, since with a given current, E. M. F 
varies directly as resistance, if the E. M. F. of the bat- 
tery be represented by E and that of the generator by 
E\ the resistance of the battery circuit by R and that 
of the generator circuit by R' ^ then ^ -. R' -. -. E -. E' . 
That is, the resistance of the battery circuit is to the re- 
sistance of the generator circuit as the E. M. F. of the 
battery is to the E. M. F. of the generator, and the 
calibration gives this E. M. F. in volts. 

A switch is shown in front by which connection can 
be made with either of two separate circuits, the right- 
hand contact, marked -^-^ to indicate the relative meas- 
urement of E. M. F., connecting with one having ten 
times the resistance of that connected with the left-hand 
contact. At the opposite corner, in the rear, three bat- 
tery connections are arranged, the right and left ones, 
marked A and B, being each through a separate cell, 
and the central one, marked 2, through the two cells in 
series ; a plug closing whichever connection is to be 
used. When the switch is on contact i, as shown, and 
the plug in A or B^ the scale readings require no cor- 
rection, and should be the same with the plug in either 
hole, each cell being a check on the accuracy of the other. 
But when the plug is in hole 2, the cells being in series, 
the reading must be multiplied by 2, since the battery 
E. M. F. is doubled ; for R \ R' \ \ 2E : 2E' . 

But when a generator of low E. M. F. is to be tested, 
the switch is connected with the contact marked yV> 
which includes, in the battery circuit, a resistance of ten 



142 DYNAMIC ELECTRICITY AND MAGNETISM, 

times that included by contact i ; hence, since the bat- 
tery current with this resistance is only -^^ of what it 
was with the former resistance, -^ the E. M. F. will de- 
velop an opposing current of equal strength, giving the 
same reading, which must be divided by lo to give the 
correct E. M. F. ; for loR : i?' : : \oE : E'. 

Each cell is if inches high and f of an inch in diam- 
eter, constructed with an inverted glass cup, inclosed in 
a brass case and hermetically sealed with soft rubber 
melted into the bottom. 

The electrodes are zinc and mercury, and the fluid 
zinc sulphate and mercuric bisulphate, formed into a 
paste in which the electrodes are inclosed ; connection 
with the mercury being made by an insulated platinum 
strip which represents the positive pole. 

This cell is selected on account of the remarkable con- 
stancy of its E. M. F., and the instrument is calibrated 
for a cell temperature of 21° C, requiring a correction in 
the reading of .000367 per degree of variation above or 
below 2 1° C, which must be made by subtraction for 
the higher temperature, and by addition for the lower. 

The cells are easily removed and replaced, when 
necessary, without disturbing the connections; and being 
small, hermetically sealed, and amply protected, do not 
interfere in the least with the handling of the instru- 
ment, and can be cheaply replaced when exhausted. 

Ayrton and Perry's Spring Voltmeters and Ammeters. — 
The unreliability of electric measuring instruments con- 
structed with permanent magnets, liable to magnetic 
loss, or to variation of magnetism from the influence of 
powerful currents, and consequently requiring frequent 
recalibration, has led to improved methods of construc- 
tion, of which the spring voltmeters and ammeters of 
Ayrton and Perry are a result. Fig. 56 represents the 
ammeter, the voltmeter being of similar construction ; 



ELECTRIC MEASUREMENT. 



143 



the principle being simply the torsion of a spring by 
electromagnetic attraction. 

The current passes through a long, narrow vertical coil, 
of high resistance in the voltmeter and low resistance 
in the ammeter, within which is suspended a light soft- 
iron tube, which incloses a long spiral spring of phos- 
phor-bronze ribbon. This spring supports the tube, 
being attached at bottom to a brass cap in which the 
tube terminates, and above to a milled head which rests 
on the glass cover and is connected with the spring by a 




Fig. 56. 

vertical pin which passes through the glass ; a similar 
pin projects downward from the bottom of the brass cap 
and passes through a hole in a support below, in which 
it has a free vertical movement ; so that the two pins 
hold the spring and tube in a vertical position ; and the 
tube being shorter than the coil, its centre on a vertical 



144 DYNAMIC ELECTRICITY AND MAGNETISM. 

line is above that of the coil. To the top of the tube 
it attached a light pointer which rotates over a scale 
graduated either in volts or amperes according to the 
design of the instrument. 

When no current is passing the pointer indicates zero 
on the left of the scale, but when the current passes, the 
tube is pulled down by magnetic attraction, in oppo- 
sition to the torsion of the spring, to a distance pro- 
portional to the current's strength ; giving it a rotary 
motion by which the pointer is deflected, which indicates 
by direct readings the E. M. F. in the voltmeter, and 
the current strength in the ammeter, according to the 
respective resistance of each instrument, and its position 
in the electric circuit. 

The tube can be turned by the milled head so as to 
bring the pointer to the required position in calibrating ; 
and a reflected image of the pointer, in a mirror placed 
under it, enables the observer to determine accurately 
its position on the scale. 

A little magnetic needle, shown at the front corner of 
the base, indicates the direction of the current; but as 
such a needle is liable to have its poles reversed by 
powerful currents, a bar magnet is preferred for this 
purpose. Since the deflection of the pointer depends on 
the magnetic attraction of the tube downwards, it must 
evidently be always in the same direction, and hence in- 
dependent of the direction of the current ; so that while 
this direction may be ascertained as above, it is not 
essential to the use of the instrument that it should be 
known. 

A light movable auxiliary coil surrounds the main 
coil and is connected with it in parallel ; this can be 
moved up or down in calibrating till a position is 
reached in which its inductive influence on the main coil 



ELECTRIC MEASUREMENT. 145 

is best adapted to the construction, where it is made 
stationary. 

The case is ventilated, as shown, to prevent the ac- 
cumulation of heat generated by the current, which 
would expand the spring and produce inaccuracy. The 
usual binding-posts connected with the terminals of the 
coil are shown at the right and left, the left post being 
marked A to distinguish them in use. 

The voltmeters are usually constructed to measure 
E. M. F. ranging from 15 volts to 1000; the ammeters, 
to measure current strength ranging from -^ of an 
ampere to 600 amperes. 

Gravity Ammeters. — While springs have greater con- 
stancy than permanent magnets in the construction of 
electric measuring instruments, their constancy is liable 
to vary, or be impaired, from well-known causes, as heat- 
ing, age, and use, imperfect material, or oxidation ; but 
the force of gravity, being always known and constant, 
may be utilized in such construction to produce instru- 
ments of great constancy. On this principle the United 
^ates Electric Lighting Company constructed the am- 
meter shown in Fig. 57. 

Two pairs of electromagnets, wound with coils of 
low resistance, and having laminated soft-iron cores, are 
placed as shown ; each pair having its coils wound on 
the same core, producing consequent poles, but mag- 
netically insulated from the other pair. 

At the centre, between these magnets, is mounted a 
soft-iron armature, lightly poised on a horizontal axis, 
the end of which is seen through the circular opening, and 
having a vertical rotary movement parallel to the mag- 
nets' plane. This armature is about 2 inches long, \\ 
inches wide at each end, f of an inch at the centre, and 
1 of an inch thick ; its sides concave, and its ends con- 
vex and slotted to correct the effects of residual mag- 



146 DYNAMIC ELECTRICITY AND MAGNETISM. 

netism. A pointer, attached to its axis, indicates che 
readings on a scale above, as shown. 

When no current is passing, the armature is main- 
tained in a fixed position by one or more little weights 
attached to its lower left-hand corner, its longer axis 




Fig. 57. 

being on a diagonal line between the lower left and 
upper right-hand corner of the instrument, and the 
pointer at zero on the left of the scale. But when the 
current passes through the coils in either direction, the 
armature rotates in obedience to the electromagnetic 
force, its longer axis tending to assume a horizontal 



ELECTRIC MEASUREMENT. 147 

position, and the pointer is deflected from left to right 
in proportion to the current strength, which is indicated 
by direct reading in amperes. 

By the removal or addition of one or more of the little 
weights, the sensitiveness of the instrument may be 
varied in calibrating, as required for different ranges of 
current strength. The terminals of the coils are shown 
at the base, and holes for ventilation at the top of the 
case. 

Instruments constructed on this principle have not 
been employed to any great extent as voltmeters, not 
being sufficiently sensitive for the light currents required. 

Since the weight, as it rises, recedes from the vertical 
line which passes through its axis of rotation, the force 
opposing rotation increases in the direct ratio of the 
increase of leverage thus produced. Hence, as equal 
divisions of the scale would represent unequal increments 
of current strength, they should be made in the inverse 
ratio of this increase of leverage. 

But as it is difficult to mark off such short spaces with 
the requisite accuracy, a gravity ammeter has been con- 
structed by the Western Electric Company, with a ver- 
tical electromagnet having a pole-piece so curved that 
the rotating armature, as it rises, constantly approaches 
it, the magnetic attraction increasing in the same ratio 
as the leverage, so that equal divisions of the scale 
represent equal increments of current strength. 

The Cardew Voltmeter. — The instruments thus far de- 
scribed are designed to be used with direct currents, 
and are liable to errors arising from self-induction in 
addition to those from the other causes mentioned. But 
since, according to a well-known law, the heat devel- 
oped in an electric conductor is in direct proportion to 
the square of the strength of the current passing 
through it, instruments can evidently be constructed on 



148 DYKTAMiC ELECTRICITY AND MAGNETISM. 

this principle which will measure either current strength 
or difference of potential, produced either by direct or 
alternating currents, and are not liable to variation from 
any of the causes mentioned. Among these the volt- 
meter, patented by Cardew in 1886, has a prominent 
place. Its operation depends on the expansion of metal 
produced by the electric development of heat. 

Fig. 58 gives a front view of this instrument and Fig. 
59 a rear view, showing its internal construction. A fine 




Fig. 58. 

platinum wire, 8 feet long, is stretched in four lengths 
in a horizontal tube, by attachment to a metal frame 
and pulleys, as shown at a, a^ t, t in Fig. 59. This tube 
is made of very thin metal, one third of its length being 
iron and two thirds brass, to maintain constancy of 
length between the points of attachment of the wires by 
such a mode of connection as to produce compensation 



ELECTRIC MEASUREMENT. 



149 



by the unequal expansion of the two metals ; and the 
horizontal position is given it to maintain constancy of 
temperature, and prevent the unequal expansion, from 




Fig. 59. 



convection of the air to which the tube and wire would 
be liable in a vertical position. 

The wire has a resistance of about 240 ohms, and at- 
tains a maximum temperature of about 200° C; and its 
expansion varying in a certain definite ratio dependent 
on the difference of temperature caused by the passage 
of the electric current, which, as stated, varies as the 
square of the current's strength, produces a variation 
in length proportional to the E. M. F. by which the 
current is generated. This produces a rotation in the 
pulley ii\ to the axis of which the pointer shown in Fig. 
58 is attached, vrhich moves in the same direction as 



1^0 DYNAMIC ELECTRICITY AND MAGNETISM. 



watch-hands when the E. M. F. increases, and in the 
opposite direction when it decreases. 

This instrument should be calibrated for the average 
temperature of the room in which it is to be used. 

The Edison Current-Meter.— Instruments for measuring 
the amount of electric current used by a consumer of light 
or power are constructed on various principles. Among 
these is the Edison current-meter, in which a small per- 
centage of the current is passed through two cells con- 
taining amalgamated zinc plates immersed in a solution 
of zinc sulphate. Zinc is thus deposited on the plates, 
which are removed and weighed at stated times, and 
the consumption of current being in proportion to the 
amount of deposition, according to the principle dis- 
covered by Faraday, is estimated accordingly. 




Fig. 6o. 
The Forbes Coulomb-Meter.— Meters like the Edisoii 



ELEC TRIG ME A S UREMEN T. 



15 



cannot be used for the measurement of alternating cur- 
rents; but one has been invented by Forbes, operated 
by the heat developed by the current, which can meas- 
ure either direct or alternating currents. Its construc- 
tion is shown in Fig. 60. The current passes through a 
flat coil of iron wire, above which is mounted, on a 
paper cone having a jeweled bearing at its apex, a mica 
disk, with mica vanes attached. The heat developed 
by the current produces an ascending current of air 
which rotates the disk, operating a light train of 
clock-work which moves indexes over two dials, regis- 
tering the current consumption in coulombs; units 
being registered on one dial and tenths on the other. 
A glass shade protects the apparatus from external 
air-currents. 

Voltameters. — Instruments like the Edison current- 
meter are more generally known as voltameters, a name 
given them by Faraday, who first proposed this method 
of electric measurement. They may be constructed 
with any substance practically susceptible of electrolysis, 
in accordance with Faraday's law that the amount of 
an element liberated by electrolysis in a given time is 
proportional to the strength of the current employed. 
Salts of copper and of silver are both employed for this 
purpose, also acidulated water. 

The Water Voltameter. — This is simply a common de- 
composing instrument in which the liberated elements, 
oxygen and hydrogen, are collected in the same receiver, 
which is graduated in cubic centimeters or any other 
convenient standard. The amount of each gas pro- 
duced at a standard temperature and pressure, by a 
coulomb of electricity, being known, the entire number 
of coulombs consumed in a given time can easily be as- 
certained. This amount, at temp. 0° C. and press. 760 
millims., is found to be 0.0579 cubic centims. of oxygen 



152 DYNAMIC ELECTRICITY AND MAGNETISM 

and O.I 15 7 of hydrogen, making 0.1736 c.c. of both, per 
coulomb of electricity. 

The use of such an instrument is confined to the labo- 
ratory, as the wasteful consumption of current, the re- 
sistance due to polarization, and the loss from recombi- 
nation of the gases, or escape of the hydrogen, renders 
it unsuitable for practical measurement. 

The Weber-Edelmann Electrodynamometer. — This in- 
strument, invented by Weber and improved by Edel- 
mann, is constructed on the principle of the deflection 
of a coil, in opposition to the torsion of a wire, by the 
joint product of E. M. F. and current strength. 

Fig. 61 shows the construction. Two coarse wire 
coils of low resistance are mounted parallel to each 
other on a stand, on three transverse brass rods, sup- 
ported by a vertical brass ring, at the centre of which 
is suspended a small, fine wire coil of high resistance; 
its plane, when at rest, being at right angles to the planes 
of the larger coils. A small plane mirror is attached 
to the centre of the small coil, to which a ray of light 
from a lamp is admitted through an aperture in the lit- 
tle screen shown in front of it. 

The suspension of the small coil is by means of a 
wire connected with its terminals and inclosed in the 
vertical brass tube shown. This wire is attached to the 
projecting rods seen at the top of the upper section of 
the tube and the bottom of the lower section ; the set- 
screws and nuts shown being used to give proper adjust- 
ment to the coil and tension to the wire ; the terminal 
rods passing through movable disks for this purpose. 

The current from the generator enters by one of its 
circuit terminals, attached to a binding-screw at the 
bottom of the lower section of the tube, passes up 
through the inclosed wire and traverses the small coil, 
goes thence through the upper section of the wire and 



ELECTRIC MEASUREMENT, 



153 



returns by the upper section of the tube to the ring, 
passes through one of the rods to a terminal of one of 
the larger coils, traverses that coil and returns by 



f 




Fig. 61. 



154 DYNAMIC ELECTRICITY AND MAGNETISM. 

another rod to the other large coil, and traversing it, 
passes out by a binding-screw to the generator through 
the other terminal of the external circuit. 

Proper insulation and connections are provided be- 
tween the rods, coils, and supporting ring to insure the 
passage of the current as above ; and its direction may- 
be reversed by reversing the connection with the ex- 
ternal circuit. 

The current in the three coils has practically the same 
E. M. F., but the difference in resistance gives the high- 
resistance coil small current strength and the low-re- 
sistance coils large current strength, so that the current 
of the small coil represents chiefly E. M. F., and that of 
the larger coils, current strength. 

When the current passes, its combined effect in the 
three coils, as represented by the product of the small 
current into the large, or E. M. F. into current strength, 
tends to bring the plane of the small coil into a position 
parallel to that of the other two; the amount of deflec- 
tion being indicated on a scale by a ray of light reflected 
from the little mirror, and observed through the aper- 
ture shown just above the ring. As this deflection 
represents the product of the E. M. F, into the current 
strength, the voltage into the amperage, it shows the 
electric power of the current as indicated in watts ; 
hence the instrument is appropriately named electro- 
dynamometer or electric-power-measurer. It can be used 
either with the direct or the alternating current, and is 
especially adapted to the latter, having no magnetic 
needle. 

Measurement of Electric Resistance. — Since current 
strength depends on the mutual relations of electro- 
motive force and resistance, it is evident that apparatus 
for varying resistance by the introduction or withdrawal 
of a definite known quantity, and of ascertaining and 



ELECTRIC MEASUREMENT. I 55 

measuring it when unknown, in order to properly adjust 
these mutual relations, is a matter of the highest im- 
portance in electrical construction. Resistance may be 
varied, as already shown, by varying the length or 
diameter of the conductor, or by changing the circuit 
from series to parallel or the reverse ; but as this usually 
requires permanent construction, it becomes necessary 
to have also some simple means by which a resistance 
of known amount can be promptly introduced into any 
circuit or withdrawn from it without interference with 
the permanent construction : this is furnished by the 
resistance coil, or rheostat as it is also termed. 

Resistance Coils. — Resistance coils are made of ger- 
man-silver wire on account of its high resistance, which 
is usually about seventeen times that of pure copper, and 
calibrated as to gauge and length for a given number of 
ohms resistance, the wire being properly wrapped for 
insulation. Fig. 62 gives an ideal view of the construc- 
tion, 

X, V, and Z are short blocks of brass, insulated from 
each other above, but connected below through the 
coils c and d, as shown ; each 
coil being wound with a 
double strand to reduce self- 
induction. Two brass plugs, 
a and d, having hard-rubber 
handles, fit into holes between 
the blocks so that when 

placed as shown, the three 
ui 1 • ^ I ' Fig. 62. 

blocks are in electric connec- 
tion, and having practically no resistance, a current 
would pass directly through them, without traversing 
the coils. But if a plug, as a, is removed, the current 
between X and V must then pass through the coil c. 
In like manner if plug d is removed, the current between 




156 DYNAMIC ELECTRICITY AND MAGNETISM. 



Y and Z must pass through the coil d\ which, being 
twice the length of c^ would have twice the resistance if 
made of wire of the same gauge, or four times the re- 
sistance if also the cross-section of the wire were one 
half that of c. In this way resistance can be varied to 
any practical extent required. 

Sets of resistance coils, calibrated for resistances vary- 
ing from T ohm or less to 10,000 or more, are con- 
veniently arranged in cases, as shown in Fig. 63. The 




Fig. 63. 

case has a hard-rubber cover by which the brass blocks 
are insulated above, each pair being connected through 
a coil below, as shown in Fig. 62. A hole in the centre 
of each block receives each plug when removed from 
between the blocks, to prevent its being mislaid, and 
connection with the electric circuit is made through the 
binding-posts shown at the right. 

To introduce any required resistance it is only neces- 
sary to remove the plug from its place between the 
blocks opposite which the resistance required is marked 
on the cover, the other plugs all remaining connected. 
If, for instance, i ohm resistance is to be introduced, let 



ELECTRIC MEASUREMENT. 157 

the first plug at the front right-hand corner be re- 
moved, opposite which " i ohm" is marked; the current 
must now flow through that coil, and pass by all the 
other coils, through the blocks and plugs; if 50 more 
ohms are to be added, the last plug at the rear left-hand 
corner is removed, opposite which is marked " 50 ohms;" 
and the resistance then becomes 51 ohms. 

The Wheatstone Bridge. — The Wheatstone bridge is an 
instrument for measuring an unknown resistance by- 
comparison with a known resistance. Fig. 64 gives an 
ideal view of its construction. Let A^ B, Cy D be four 
wires connected at the points F, Q, M, N, and let M and 
N be connected with the galvanometer G, and F and Q 
with the battery X, by which a current can be sent from 
F to Q. This current will divide at F^ and the portion 




Fig. 64. 
passing through each branch of the circuit will be in- 
versely proportional to the respective resistance of each. 
Now it is found that the potential between any two 
points in an electric circuit varies inversely as the re- 
sistance between them; and as the E. M. F, between 
any two points is represented by their potential differ- 
ence, the E. M. F. at M would vary as the ratio of re- 
sistance in C to that in F>, and the E. M. F. at Was the 



158 DYNAMIC ELECTRICITY AND MAGNETISM. 

ratio of resistance in A to that in B\ if these ratios are 
equal, then the E. M. F., or electric pressure at J/, is 
equal to that at N^ irrespective of the amount of current 
in each branch, and no current can pass between these 
points, and hence there can be no deflection of the gal- 
vanometer needle. But if either ratio differs from the 
other, then current will pass between M and JV in pro- 
portion to this difference and produce deflection. 

Suppose this difference to be caused by the introduc- 
tion of an unknown resistance into the arm D; then by 
varying the resistance in B till the deflection disappears, 
equality between the ratios is restored, and as theresist- 
ances of A, B, and C are known, that of D may be com- 
puted; for, allowing the letters to represent the resist- 
ances, 

Since C: D::A:B, AZ> = BC, and D = ^, 

A 

In like manner, v^^hen the respective resistances of any 
three of the arms are known, that of the fourth may be 
ascertained. 

The total resistance or total current in either branch, 
or the equality or inequality of resistance or current in 
the arms, are matters of indifference, equality of ratios^ 
as above, being the principle of construction. 

As the potential decreases from F \.o Q \n both 
branches of the circuit, it is evident that if an unknown 
resistance greater than that of D were substituted for 
X^'s resistance, the effect would be to reduce the poten- 
tial difference, or E. M. F., between Cand Z>, producing 
deflection of the needle by a flow of current from N to 
M, and requiring proportional increase of resistance in 
B to restore the equilibrium. But if this unknown 
resistance were less than that of D, the effect would be 
to increase the potential difference between CandZ>, 



ELECTRIC MEASUREMENT. 159 

producing deflection by a flow of current from M to iV, 
and requiring proportional decrease of resistance in B. 
This instrument may be constructed in any convenient 
form in which the mutual relations of the different 
parts to each other are properly maintained; and sets of 
resistance coils may be so connected with the different 
arms as to vary the resistance as required. Fig. 65 
shows a convenient, practical form. 

A Q C M D P B 



On an insulating strip of hard rubber are mounted 
five copper strips furnished with binding-screws; and 
between the two end strips is stretched a wire, connected 
with them, made of a compound metal composed of 85 
parts platinum and 15 parts iridium, having high resist- 
ance and not easily oxidized; and parallel to it is a 
graduated scale on which the resistances of equal 
divisions of the wire are marked in ohms, after proper 
calibration. The arms and connections for the battery 
and galvanometer are lettered in the cut to correspond 
to the lettering in Fig. 64. The arm A extends from Q 
round to JV, including a section of the resistance wire, 
and the arm B from F round to JV, including the re- 
maining section; arm C, from Q to M, and arm B, from 
M to F : the battery connections being at F and Q, and 
the galvanometer connections at M and N. The con- 
nection at N IS made with a slide, mounted on the re- 
sistance wire, to which is attached a pointer which 
indicates on the scale the amount of resistance included 
in each of the arms A and B. The unknown resistance 
which is to be measured can be inserted either at Cor 



l60 DYNAMIC ELECTRICITY AND MAGNETISM. 

D, as preferred, the remaining space being then filled 
with a known resistance. 

When deflection of the needle is produced by the in- 
sertion of an unknown resistance at either of those 
points, a movement of the slide, either to the right or 
left as required, changes the relative resistances of the 
arms A and B, and restores the equilibrium by making 
the ratio of resistance between A and B equal to that 
between C and D\ and the value of the former ratio 
being indicated on the scale, the value of the unknown 
resistance can be ascertained, as already explained. 

Keys are provided in the battery and galvanometer 
circuits by which each circuit can be opened or closed 
as required; the battery circuit being always closed first 
and opened last, to avoid the violent oscillation of the 
needle due to the extra current produced by self-induc- 
tion on opening or closing a circuit. 

Fig. 66 shows a very elaborate instrument, combining 
the galvanometer and a set of resistance coils, by which 
resistances from one hundredth of an ohm to a million 
ohms or more can be measured. 

The resistance to be measured is connected with the 
two binding-posts on the left, the battery with the two 
on the right. Resistance coils ranging from o to 10,000 
ohms are arranged in four rows of ten each, marked 
respectively " units," "tens," "hundreds," and "thou- 
sands;" and in front of the galvanometer are two rows, 
A and B^ of three each, the corresponding ones on each 
side marked respectively " 10," " 100," and " 1000.' 

In the long rows, each of the ten coils in the same 
row has the same resistance; each in units' row having 
one unit, each in tens' row one ten, and so on. But in 
the short rows, each coil has the resistance marked on 
its bolt. The coils in each long row are connected to- 
gether in series by the bolts, each coil being connected 



ELECTRIC MEASUREMENT. 



i6i 



with two bolts by its opposite ends. Parallel to each 
row of bolts and insulated from them is a brass bar, 




having practically no resistance; and each of the three 
bars, marked *' units," ''tens," and " hundreds," is elec- 



1 62 DYNAMIC ELECTRICITY AND MAGNETISM. 

trically ccnnected underneath, at the left, to the row of 
bolts in front of it by the bolts marked o. 

When plugs are placed in each of the four holes at 
the left, opposite the bolts in the four long rows marked 
o, the current passes directly through the four bolts, 
plugs, and ends of the bars thus connected, without 
passing through any of the coils; but if a plug is 
removed to the right, then the current must pass 
through all the coils to the left of it in that row and 
introduce the resistance indicated by the number on the 
bolt and the word on the connected bar in front of it. 
For instance, if a plug connects units' bar with bolt 4, 
as shown, the current passes through coils i, 2, 3, and 4, 
introducing four units of resistance; in like manner the 
plug connecting bolt 6 with tens' bar introduces 6 tens, 
bolt three connected with hundreds' bar 3 hundreds, 
and bolt 7 connected with thousands' bar 7 thousands, 
making the entire resistance introduced 7364 when the 
plugs in the two short rows are both opposite bolts 
numbered alike, as shown. 

The two bars parallel to the two short rows are con- 
nected underneath by a wire, and each coil in each row 
has a separate connection with the electric circuit ; the 
three in row A being separately connected at the same 
point with the arm corresponding to A in Fig. 64, and 
the three in row B with the arm corresponding to B. 

The four long coils connect with the arm correspond- 
ing to D ; and the resistance to be measured, with the 
arm corresponding to C. Hence if the resistance in A 
equals that in B^ and the plugs in the four long rows are 
moved to the right or left till the needle shows no de- 
flection, then the resistance in the four rows must equal 
that to be measured, since A : B : \ D \ C. Hence, with 
the plugs placed as shown, that resistance would be 
7364 ohms. 



ELECTRIC MEASUREMENT. 1 63 

But if a greater resistance than any represented by 
the four long rows is to be measured, as 100,000 ohms 
or more, then by changing the plug in row B to bolt 
10, and that in row ^ to bolt 1000, the resistance of arm 
A is made 100 times that of arm B ; hence when the 
plugs in the four long rows are moved till the needle 
shows no deflection, the resistance to be measured must 
be 100 times that indicated in the four rows, which in 
the special case given would be 736,400. But if the plug 
in row A were at too and that in row B at 10, then, the 
resistance of A being only ten times that of B^ the re- 
sistance in the above case, when the deflection was 
eliminated, would be 73,640. 

If a smaller resistance than any represented in the 
four rows is to be measured, as -^ of an ohm, then by 
placing the plug in units row opposite i, and those in 
the other three long rows opposite o in each, and 
moving the plug in row A to 10 and that in row B to 
100, the resistance in A is made -^-^ of that in B\ hence if 
the needle shows no deflection, the resistance to be 
measured is shown to be -^-^ of an ohm. In a similar 
manner, a resistance of y^o- of an ohm maybe measured. 

Hence w^e see that when the indicated resistance in 
row B is greater than in row A^ the effect is to divide 
the indicated resistance in the four rows by the ratio 
oi B to A ; but when the indicated resistance in B is less 
than that in A, the effect is to multiply the indicated 
resistance in the four rows by the ratio of A to B. In 
a similar manner any of the indicated resistances can 
be multiplied or divided. 

If, in the construction, the relative positions of arms 
Cand D are reversed, the effect is to reverse the rela- 
tive positions of arms A and B with reference to them; 
and hence the multiplication and division, as above. 

By increasing the number of coils, and range of re- 



164 DYNAMIC ELECTRICITY AND MAGNETISM. 

sistance, in both the long and short rows, within prac- 
tical limits, any required resistance, great or small, can 
be accurately measured. 

The battery and galvanometer keys, marked respect- 
ively B and G, are shown in front. In a recent form of 
this instrument the battery key is placed above the 
galvanometer key and insulated from it, so that the 
same pressure closes both, the battery key first, as re- 
quired ; and the binding-posts for the battery are placed 
at the right of the galvanometer, and those for the re- 
sistance to be measured at the left ; a units' coil is also 
added to each of the short rows. 

In another form of this instrument, the bars are 
omitted and the resistance introduced by removing 
plugs, as shown in Figs. 62 and (i:^,. 

The plugs should always be pressed in tight, to insure 
perfect contact. 



THE DYNAMO AND MOTOR, 1 65 



CHAPTER VII. 

THE DYNAMO AND MOTOR. 

The Magneto-Electric Generator. — It has been shown in 
Chapter V that transient electric currents are generated 
in a conductor forming a closed circuit, when moved 
through a magnetic field in such a manner as to cut a 
varying number of lines of force and produce a differ- 
ence of potential between different parts of the circuit ; 
and that the E. M. F. varies as the number of lines cut 
per unit of time, and the strength of the current as the 
E. M. F. divided by the resistance. It has also been 
shown that when such a conductor is in the form of a 
coil having a soft iron core, the electric development is 
greatly increased by the coefficient of magnetism in- 
duced in the core. 

On these principles the little instrument known as the 
magneto-electric machine was invented by Pixii in 1833, 
in which subsequent improvements were made by Sax- 
ton and Clarke. It consists, as now constructed, of a 
short U electromagnet, mounted on an axis, with its 
poles close to those of a permanent magnet and at right 
angles to them, and made to rotate rapidly by means of 
a crank, band-wheel, and gearing. At each make and 
break thus produced, transient, alternating currents are 
generated in the coils ; and the coil terminals being at- 
tached to two brass plates fitted to opposite sides of the 
axis, with insulating material between them, the cur- 
rents are taken up and passed to an external circuit by 
two brass springs which press against these plates. 

Commutation. — The plates being insulated from each 



1 66 DYNAMIC ELECTRICITY AND MAGNETISM. 



Other, and out of contact with the sorings during the 
break, and brought into reversed contact with them at 
the instant of current reversal, which occurs at each 
half revolution, their position with reference to the 
springs is reversed as the currents are reversed, and 
hence the currents are all made to flow in the same 
direction through the external circuit. A direct current 
made up of these transient, alternating currents is thus 
produced by commutation, with perceptible intermission 
at each make and break, its smoothness varying with the 
rapidity of the rotation. 

Improved machines of this kind were constructed by 
Siemens, Wilde, and others, among which was a very 

mi 
ii 




Fig. 67. 
powerful one, made by the Compagnie TAlliance of 
Paris, of the following construction, illustrated by Fig. 
67. 

The Alliance Machine. — Six bronze wheels, mounted 
on a horizontal shaft, carried 16 electromagnets on each 
circumference, 96 in all which rotated between 7 sets 



THE DYNAMO AND MOTOR. 



167 



of laminated steel magnets, 8 in a set, fixed radially, 
poles inward, in 8 rows, on a horizontal frame, opposite 
poles alternating both radially and lengthwise ; so that 
the core of each bobbin, as it rotated betw^een them, was 
alternately exposed to opposite poles at each end, 16 
times at each rotation, the 96 electromagnets thus 
generating 16 X 96 = 1536 transient currents ; and as 
the shaft rotated 350 times per minute, 350X1536 
= 537,600 currents per minute were generated. 

A machine with alternating current was 
employed for the electric light, for light- 
houses, and one with direct current for elec- 
tro-plating and similar work. 

The Siemens Armature. — The principal im- 
provement made by Siemens consisted in a 
new^ style of bobbin, or armature, as it was 
called, illustrated by Fig. 68, invented in 1856, 
in which the coils were wound lengthwise, 
parallel to the axis of rotation, on the flat 
central part of a long iron core between 
two flanges, each convex outside and straight 
inside, and projecting beyond the central 
part at the ends as shown ; a cross-section 
resembling the letter H. 

This armature rotated between large pole- 
pieces attached to the poles of a pow^erful 
laminated steel magnet, the tw^o flanges 
being the armature's poles, and its coils cut- 
ting across the lines of force ; and being more 
fully exposed in the magnetic field than in 
the old style of winding, the electric devel- 
opment was proportionally increased. 

Wilde's Machine. — Wilde's improvement consisted in 
substituting a pair of electromagnets for the steel mag- 
net to produce the magnetic field, and exciting them by 



Fig. 68. 



l68 DYNAMIC ELECTRICITY AND MAGNETISM. 



a small Siemens machine, mounted above it as shown in 
Fig. 69 ; the Siemens armature being used below as well 
as above. The current from the armature of the ex- 
citing machine passed in circuit through the coils of the 
electromagnets, while that from the lower armature 




Fig. 69. 

passed out through the external circuit, being made 
direct by commutation in both machines. The pole- 
pieces referred to are indicated in the cut hy m n above 
and T T below, and insulated from each other by brass 
indicated by o and i. 

The Dynamo. — Iron when magnetized always retains 
a little residual magnetism, and when wrought into any 



THE DYNAMO AND MO 2' OR, 1 69 

form acquires a similar quantity by the manipulation. 
It was proposed by Siemens and Wheatstone, in 1867, 
to excite the generator by the multiplication of this 
residual, found in the cores of the electromagnets and 
armature, by connecting the electromagnet coils with 
the armature circuit, and thus dispense with the exciting 
machine. The method of doing this may be illustrated 
as follows: 

In Fig. 69, the magnet coils are connected together 
below, and have their terminals at/ and q above; if the 
exciting machine be removed and one of the circuit 
terminals below, as that on the right, be connected with 
the coils at ^, and the other, after passing through the 
external circuit, be connected at/, then a current pass- 
ing from the armature out through the left-hand termi- 
nal, and traversing the circuit, must return to the right- 
hand terminal by way of / and ^, through the magnet 
coils, and thence through the armature coils to the 
left-hand terminal. 

The armature of a new machine, so constructed, being 
put in rotation for the first time, the incipient current 
generated in its coils during the first half-revolution, by 
the residual magnetism of the cores, passing through 
the magnet coils as above, increases this residual, which 
by its reaction increases the current in the armature 
coils in like ratio. At the next half-revolution these 
increased effects are doubled by the mutual reaction; 
and this doubling occurring at each subsequent half- 
revolution and being repeated several thousand times 
per minute by the rapid rotation of the armature, the 
current, thus continually increasing in geometrical ratio, 
rises in a few moments to its full normal force, limited 
by the magnetic saturation of the cores and the carry- 
ing capacity of the coils. 

The machine, constructed on these principles, was 



170 DYNAMIC ELECTRICITY AND MAGNETISM. 

designated as the dynamo-electric^ in distinction from the 
magneto-electric, and subsequently became known 
briefly as the dynamo. 

The electromagnets producing the field were called 
the field-magnets^ in distinction from the armature, which 
is also an electromagnet. The springs for taking up the 
current were called the brushes ; each consisting of a 
number of thin copper plates projecting beyond each 
other at the contact end and soldered together at the 
outer end. And the pair of insulated segments with 
which they made contact, and by which the commuta- 
tion was produced, was called the commutator. 

Hence the essential parts of the direct-current dyna- 
mo became known as the armature, t\v^ field-magnets, the 
commutator, and the brushes. 

Ladd's Machine. — The current of the machine first 
constructed by Siemens, in 1867, alternated automati- 
cally between the internal and external circuits, being 
diverted from the latter when employed to excite the 
former. During the same year a machine was con- 
structed by Ladd, in which the current through both 
circuits was, made continuous. It was substantially the 
same as the Wilde, with the steel magnet removed, the 
two armatures retained, one being connected with the 
magnet coils and the other with the external circuit, 
and the magnets placed in a horizontal position between 
armatures of equal size, and supported at each end on 
large vertical pole-pieces. 

The Pacinotti-Gramine Armature. — An armature having 
the form of a wide ring was invented by Pacinotti in 
1862, in which the coils were wound between projec- 
tions on an iron core. An improvement on this was 
made by Gramme in 1870, illustrated by Fig. 70, in 
which the core was composed of annealed iron wires and 
entirely covered with the coils, only a few of which are 



THE DYXAMO AND MOTOR. 



171 



shown in the cut; the winding being continuous from 
coil to coil as shown. 

The covering of the core in this manner does not 
materially obstruct the transmission of magnetic force, 
copper being diamagnetic, so that such a core is prac- 




\ YV 




H 


'^ xT 



Fig. 70. 



tically as susceptible of magnetism as that of the 
Siemens armature. 

Improved Commutator. — An improved style of commu- 
tator was also invented, and used by Gramme in the 
construction of his dynamo in 1870, in connection with 
his improved armature. It is shown in cross-section in 
Fig. 70, and consisted of a number of short copper bars 
mounted on one end of the armature's axis, parallel to 
its length, and insulated from it and from each other by 
wood or other insulating material, filling the spaces 
between them and forming a cylinder under them on 
the axis. Each bar is attached to a coil as shown, so 
that the number of coils and bars is equal. 

As the currents reverse at each half revolution, a com- 
mutator having but two segments produces an inter- 
mittent current, as has been shown; but if it have four 
segments, as shown in Fig. 71, the brushes are brought 
into contact with two of the segments at each quarter 
revolution, and if each brush make contact with the 
approaching segment before breaking contact with the 



172 DYNAMIC ELECTRICITY AND MAGNETISM. 

receding segment, so as to bridge the intervening space, 
no intermission can occur. 

But as the coils, at each revolution, cut a varying 
number of lines of force per unit of time in different 
parts of the field, each alternate current must rise with 
the increase and fall with the decrease of magnetic 
force ; hence, with only four segments, the current, 







Fig. 71. 

though continuous, would be uneven, but with eight 
segments, as shown in Fig. 70, it becomes at a high 
speed of the armature practically even, being made still 
more even as the number of segments is increased. 

It is evident that the current cannot pass from one 
segment to another without traversing all the convolu- 
tions of the intervening coil; and as each convolution 
adds its quota to the current, and each coil is connected 
with the adjoining coil, all the currents thus generated 



THE DYNAMO AND MOTOR. I /J 

combine to augment the volume of current flowing 
through the outer circuit. 

Direction of the Current. — If the armature, shown in 
Fig. 70, rotated in the direction of watch-hands and 
the current, transmitted from it through the field-mag- 
net coils, should circulate in such direction as to induce, 
in their cores, a north pole on the right of the armature 
and a south pole on the left. Then, according to the 
principles of electromagnetic induction explained in 
Chapter V, the currents generated on the outside part 
of the right-hand coils of the armature, between its 
core and the north field-magnet pole, would flow from 
the observer and be conducted back oppositely through 
the inside part, while those generated in the left-hand 
coils would flow in reverse order. And these currents, 
collected and made direct by the commutator, would 
enter the external circuit and field-magnet coils by the 
upper brush, and return to the armature by the lower 
brush. 

If the rotation of the armature were reversed, the 
direction of the current and polarity of the magnets 
would be reversed also. 

Interior Wire of the Gramme Armature. — Iron being 
paramagnetic, the lines of force in the magnetic field 
cannot penetrate the Gramme armature core and pass 
through the interior of the ring, but are taken up by the 
core, which thus becomes magnetized. Hence the in- 
terior and end wire of the coils does not cut those lines, 
and cannot in this manner take part in the electric gen- 
eration, but serves as a conductor of the currents gen- 
erated in the exterior wire. It also increases the electric 
generation by the coefficient of magnetic induction 
received from the core. 

According to a theory now somewhat obsolete, the 
currents are generated by the lines of force threading 



174 DYNAMIC ELECTRICITY AND MAGNETISM. 

through the coils, the interior wire thus taking part in 
the generation equally with the exterior; but experi- 
ment seems to prove that this theory is fallacious, as no 
current is found in the interior and end wire when not 
continuous with the exterior. 

In the Sperry dynamo, interior pole-pieces, parallel to 
the axis of rotation, are used to render this wire active, 
the armature rotating between them and similar ex- 
terior pole-pieces projecting from the field-magnets. 

Another common form of construction is to wind all 
the wire on the exterior, passing it around projections 
on each end of the ring. 

The Cylinder Armature. — The drum or cylinder arma- 
ture is also a common form, in which the wire is wound 
lengthwise on a cylinder, passing over the ends, as 
shown in Fig. 72. The core generally consists of a large 
number of thin sheet-iron disks, one of which is shown 
at B^ mounted on a shaft and insulated from each other 
by tissue-paper. These are usually perforated by open- 
ings which, when placed opposite each other, form 
tubes for interior ventilation, connecting with ventilat- 
ing spaces between groups of disks, as in the Weston 
armature, shown at A, on which are also projections be- 
tween which the wire is wound. They are also made 
without openings or projections, as in the Edison arma- 
ture, shown at C, the wire being confined by brass 
bands, as shown. 

This construction of the core prevents the formation 
of the Foucault currents to which solid cores are liable, 
and which heat them and serve no useful purpose. And 
the disks, being parallel to the lines of force and at right 
angles to the currents, are in the best position for 
electromagnetic induction. Armatures of the Gramme 
pattern are also constructed with cores of this kind, 
made up of flat rings instead of disks. 



THE DYXAMO AXD MOTOR. 1/5 

The core should come as close to the pole-pieces as 
possible, to insure maximum magnetic induction, and 




ABC 

Fig. 72. 
hence the wire wound on it should be evenly distributed, 
and of the minimum quantity and gauge requisite for 
proper electric induction and resistance. 



1/6 DYNAMIC ELECTRICITY AND MAGNETISM. 

Closed-Circuit and Open-Circuit Armatures. — Armatures 

wound like the Gramme, in an endless spiral, with at- 
tachment to the commutator segments by radial arms, 
at regular intervals, are known as closed-circuit arma- 
tures; and the same designation is applied to those in 
which the coils are wound separately but connected 
with each other at the commutator, as in the armature 
of the Weston dynamo, shown in Fig. 73. 




Fig. 73. 



In another style, known as the open-circuit armature, 
each coil is independent of every other, its terminals 
being connected to two opposite segments of the com- 
mutator which have no connection with the other coils. 



THE DYNAMO AND MOTOR. 



i;; 



as in the armature of the Brush dynamo; hence only 
those coils connected with the brushes through the 
commutator are in action simultaneously, each set 
coming into action as the other set passes out. Four 
brushes are employed in an eight-coil Brush dynamo, 
and the contacts are made in such a manner that six 
coils are in action simultaneously. 

Location of the Armature's Magnetic Poles. — In accord- 
ance with the principles of magnetic induction, the 
polarity induced in the core of the armature by the 
field-magnets during rotation is opposite to that of the 
inducing poles, as shown in Fig. 74. But this polarity 



'^'i7¥B 




Fig. 74. 

is comparatively weak, the core's most effective polarity 
being that induced by the currents circulating through 
the armature's coils, the tendency of which is to induce 
similar poles in proximity to those of the field-magnets 
wliich, by mutual repulsion, are deflected into the posi- 
tion indicated hy n n and j- i- on a line joining the brush 



1 7^ DYNAMIC ELECTRICITY AND MAGNETISM. 

contacts; each half of the core, divided on this line, be- 
coming a separate magnet. 

The poles of the field-magnets are deflected in the 
opposite direction, the north pole to the lower corner of 
the pole-piece on the right, and the south pole to the 
upper corner of the pole-piece on the left; a line join- 
ing their centres being nearly at right angles to that 
joining the stronger armature poles. Hence the lines 
of force become contorted as shown. 

Magnetic Lag. — The armature core does not become 
fully magnetized at the instant induction occurs, nor 
fully demagnetized at the instant it ceases; an infinites- 
imal moment being required for its saturation in the 
first instance and its demagnetization in the second, 
known as magnetic lag, during which its poles are car 
ried slightly forward in the direction of the rotation; 
this tends to separate the dissimilar poles induced by 
the field-magnets from the field-magnet poles, and thus 
to increase the contortion of the lines. 

Position of the Brushes. — The brushes make contact 
with the commutator on or near the neutral line on 
which the currents reverse, as shown in Fig. 74, and 
where consequently no currents are generated; hence, 
in a closed-circuit armature, the parallel currents gen- 
erated on the left pass out from the armature by the 
upper brush, as each segment of the commutator comes 
into contact with it, and those generated on the right 
are added to the inflowing current entering the armature 
by the lower brush. 

If the brushes were shifted into the line of highest 
potential, which is at right angles to the neutral line, 
the wire in which the parallel currents are generated, on 
either side, would be carried round by rotation to the 
opposite side before the connecting commutator seg- 
ments reached the brush, and the currents neutralized 



THE DYXAMO AND MOTER, 179 

by opposing currents generated in the wire, and the 
external current cease. 

But if the brushes made contact on a line between 
the neutral line and line of highest potential, a partial 
neutralization by opposing currents would occur, and 
tlie electric potential vary as the distance of the brushes 
from the neutral line; increasing as they approached 
it and decreasing as they receded from it. By shifting 
the brushes in this manner, automatically or otherwise, 
the potential and resulting current can be varied and 
regulated as required. 

Such regulation is common, but its range is limited, 
and it cannot always be used advantageously, as it tends 
to increase sparking at the brushes, a wasteful and in- 
jurious heating effect, difficult to suppress entirely. 

The Field-Magnets. — The field-magnets of different 
dynamos vary greatly in construction and constitute 
the principal part of the framework of each machine, 
and hence they are so constructed as to support the 
various parts in the most convenient manner and give a 
compact, appropriate form, without interference with 
their special function. 

They have massive cores, usually of the best cast-iron, 
preferably annealed, malleable iron, though wrought- 
iron is also employed, but the advantage is not usually 
sufficient to compensate the extra cost. These cores 
should be sufficiently massive to insure the absorption 
of all the magnetism which can be generated in them 
without over-saturation. They terminate, at one end, 
in enlarged pole-pieces which nearly inclose the arma- 
ture, the opposite ends being connected by a cast-iron 
yoke, or bolted together by cross-bars, to complete the 
magnetic circuit. They are wound with heavy insulated 
cr)pper wire, the winding being continuous from core to 
core. 



l8o DYNAMIC ELECTRICITY AND MAGNETISM. 

A single pair of such magnets may be employed, or 
two or more pairs, each core having a separate pole- 
piece, or two or more cores being joined to the same 
pole-piece. 

Series, Shunt, and Compound Winding. — There are 
three principal methods of winding the field-magnets, 
known respectively as the series^ the shunt, and the com- 
pound winding. 




Fig. 75. 

In the series method, illustrated by Fig. 75, the entire 
current traverses a single route of low resistance, pass- 
ing in series through the armature, the field-magnets, 



THE DYNAMO AND MOTOR, 



I8 



and the external circuit; so that any variation of resist- 
ance, at any point, affects the entire series equally. 

In the shunt method, illustrated by Fig. 76, the cur- 
rent traverses two distinct routes; dividing, at the upper 
brush, in the inverse ratio of the resistance of each cir- 
cuit. The main current flows to the right through the 




Fig. 76. 

coarse wire of the external circuit, while a small cur- 
rent, varying from 1.5^ to 20^ of the entire volume, 
flows through the shunt, or fine wire with which the 
magnets are wound, and is employed exclusively to 
excite them. 

If the resistance of the main circuit is increased, the 
strength of its current is proportionally diminished. 
But the potential difference, or E. M. F., between the 
brushes, representing the electric pressure, is increased 



1 82 DYNAMIC ELECTRICITY AND MAGNETISM, 



by the diminished flow of current in the ratio this in- 
creased resistance bears to itself plus the armature's resist- 
ance: and as the resistance of the shunt remains constant, 
the strength of its current is proportionally increased by 
this increase of E. M. F.: and the magnetism of the 
core being increased in the inverse ratio of its satu- 
ration, by this increase of current strength in the shunt, 
its reaction increases the current strength in both cir- 




FiG. 77. 
cuits; thus supplying electric energy to overcome the 
increased resistance. By this series of adjustments an 
equilibrium between these various factors is established, 
the total electric energy developed, varying as the me- 
chanical energy expended. Decrease of external re- 
sistance reverses these results. 

The resistance of the shunt may be varied as re- 
quired, by resistance coils. 



THE DYNAMO AND MOTOR. 183 

The compound winding, illustrated by Fig. 77, is a 
combination of the series and shunt methods; a shunt 
wire of high resistance, used only to excite the magnets, 
being employed in connection with the low resistance 
wire, which is wound by the series method and excites 
them also. The automatic regulation is similar to that 
of the exclusive shunt method, except that the entire 
current flows through the magnet coils. 

Each of these methods of winding has its special 
adaptation to the requirements of a certain kind of 
work; as, for instance, in electric lighting it is found 
that the series-wound machine is usually the most suit- 
able for arc lighting, and the shunt and compound 
wound for incandescent lighting; arc lighting requiring 
high E. M. F. and comparatively small current, while 
the requirements for incandescent lighting are the re- 
verse; which leads to the classification given below. 

Constant Current Dynamo. — To maintain a number of 
arc-lamps, connected in series, at a given illumination, 
a constant current of ten or more amperes, flowing from 
lamp to lamp, is required for each. If but one lamp 
were lighted, the required E. M. F. or potential would 
be comparatively small; but if two lamps were lighted, 
the resistance being doubled, the E. M. F. must be 
doubled to maintain the same current strength; and the 
same ratio of E. M. F. to resistance must be maintained 
for any number lighted or extinguished. 

Hence the construction and regulation of a dynamo 
for this work, or any work having similar requirements, 
must be such as to furnish E. M. F. capable of variation 
within the required range; and a machine so constructed 
is known as a constant-current dynamo, and is usually 
series-wound as stated above. 

Constant-Potential Dynamo. — But if the required work 
were the maintenance at a given illumination of a 



184 DYNAMIC ELECTRICITY AND MAGNETISM. 

number of incandescent lamps connected in parallel, 
the lamps being on branches derived from the main 
circuit, the variation of resistance is confined to these 
branches, in which it becomes adjusted to the require- 
ments of the current, the resistance of the main circuit 




Fig. 78. 



remaining constant; hence the E. M. F. remains nearly 
constant; and a machine adapted to such work, or work 
having similar requirements, is k lown as a constant-paten- 
tial dynamo, and is either shunt or compound wound. 



THE DYNAMO AND MOTOR. 1 85 

The Edison Dynamo. — Dynamos differ greatly in ap- 
pearance and minor details of construction, but tlieir 
general construction and the relations of the different 
parts will be readily understood from the Edison 
dynamo, shown in Fig. 78, which is a direct-current, 
shunt-wound machine, used especially for incandescent 
lighting, and a fair representative of its class. 

The field-magnets, mounted vertically, rest on mas- 
sive pole-pieces inclosing the armature below, and on 
their left are shown the connections of the coils, the 
lever above by which the external circuit, represented 
by the projecting terminals, is connected and discon- 
nected, the projecting end of the armature below, with 
the commutator and brushes, the latter attached to a 
yoke, movable manually for adjustment of potential. 
The oil-cups, band-wheel, and screws for shifting the 
machine's position, to tighten or loosen the belt, are also 
shown below, and the lamp above, which indicates the 
general state of the current. 

Alternating Current Dynamos. — The transient currents 
generated by the armature, when passed into the ex- 
ternal circuit without commutation, produce a continu- 
ous alternating current, and electromagnetic machines 
having such construction are known as alternating cur- 
rent dynamos. 

The Gordon Dynamo. — The older machines of this class 
have a construction somewhat similar to that of the 
Alliance magneto-electric machine, already described; 
electromagnets with alternating poles taking the place 
of the steel field-magnets. The Gordon machine is of 
this construction; 64 short field-magnets being mounted 
transversely on the circumference of a wheel which 
rotates between two stationary armatures of similar 
construction, each having 64 coils; and the coils being 
oppositely wound on each alternate bobbin, both in the 



1 86 



DYNAMIC ELECTRICITY AND MAGNETISM. 



armature and field-magnets, produce alternating poles 
in each. 

The currents flow from the armature coils to the 
external circuit without the intervention of a collector 
and brushes; and the field-magnets are excited by two 
direct-current dynamos. 

The Westinghouse Dynamo. — The Westinghouse alter- 
nating-current dynamo represents an improved method 




Fig. 79- 

of construction, the principal features of which have 
been adopted by several machines of this class. 

Fig. 79 is a sectional view of the machine, as seen 
from the end of the armature shaft, representing i6 



THE DYNAMO AND MOTOR. 



187 



field-magnets attached radially to a circular frame, their 
opposite, alternating poles inclosing a central space in 
which the armature rotates; their cores and winding 
being shown in section above. 

Fig. 80 is a sectional view parallel to the shaft; a side 




Fig. 80. 
view of one of the field-magnets being given below, and 
that of a core above. Mounted on the shaft at the left 
of the armature is the collector, composed of two copper 
rings, insulated from each other, on each of which a 



1 88 DYNAMIC ELECTRICITY AND MAGNETISM. 

brush, connected with a separate terminal of the external 
circuit, makes contact. 

The armature core is composed of insulated sheet- 
iron disks, and ventilated by tubular openings in the 
manner already described; and the coils are wound in 
a single layer on its external surface and looped around 
projections at the ends. The manner of winding is 
shown in Fig. 8i, a correct idea of it being obtained by 




Fig. 

supposing the coils to lie at right angles to the surface 
of the paper, the outer ends turned from the observer 
and the inner ends towards him. Each alternate coil is 
oppositely wound as shown, and they all form a contin- 
uous closed circuit, the opposite terminals of which are 
connected with the separate rings of the collector; the 
current passing out from one ring and returning to the 
other alternately. 



THE DYNAMO AND MOTOR. 1 89 

Separate Excitation. — The direct current, always re- 
quired for exciting the field-magnets, in the Westing- 
house and similar dynamos, is obtained, as in the Gordon, 
from a separate, small machine. This separate excita- 
tion, which involves extra expense, complication, and 
inconvenience, may be avoided by the generation of a 
separate, direct current in the machine itself by the 
commutation, for this purpose, of a small portion of the 
alternating current. But separate excitation is found 
to be the most practicable for the large dynamos usu- 
ally employed for alternating-current work. 

Advantages of the Alternating Current Dynamo. — The 
peculiar construction of the alternating-current dynamo 
and the elimination from it of the commutator, with its 
resistance and wasteful sparking, results in the genera- 
tion of currents of much higher potential, with less 
internal resistance than it is possible to obtain from the 
direct-current dynamo. Such currents can overcome 
the resistance of the external circuit more efficiently 
than those of low potential; and on this principle is 
based the practical rule that the amount of copper in 
the conductor should vary inversely as the square of 
the E. M. F. ; according to which it is found possible to 
transmit such currents to points remote from the gen- 
erator by comparatively small wires, and thus distribute 
electric energy, for practical use, over a much larger 
area, at the same cost, than is possible with the direct 
current system; or over the same area at far less cost. 

This economical advantage is increased where elec- 
tricity can be generated more cheaply, as by water- 
power, at a point remote from where it is required for 
consumption; or where the generating station can be 
located on cheap property to furnish current for use on 
more expensive property, as often happens in cities. 

The Converter. — Incandescent lighting is the principa. 



190 DYNAMIC ELECTRICITY AND MAGNETISM, 

ase for which the alternating current is now employed; 
and as this requires a large current distributed in small 
parallel currents among a great number of lamps, as 
explained in Chapter XI, 5000 being sometimes thus 
illuminated by a single dynamo, the conditions of high 
potential and comparatively small current, under which 
the electric energy is delivered, require to be reversed 
at the several points where it is to be consumed. 

This is done by the apparatus known as the converter 
or transformer, which is simply an inverted induction- 
coil of special construction; the primary coil consisting 
of fine wire which receives the high potential current 
from the dynamo, and the secondary coil, insulated 
from the primary, consisting of coarse wire in which, in 
consequence of the low resistance, a large current is in- 
duced and supplied to the lamps. 

Instead of an iron core inclosed by the coils, the coils 
are inclosed in an iron case composed of insulated sheet- 
iron plates, built up in the same manner as the armature 
cores already described; the two coils being placed side 
by side, so that both are equally exposed to the mag- 
netic induction. 

These converters, mounted on poles or otherwise, are 
distributed along the line between two parallel wires, 
one connected with the primary coils and dynamo, and 
the other with the secondary coils and lamps. At each 
point where light is required, a converter of the capacity 
requisite to furnish current for the required number of 
lamps is placed. Ten to eighty lamps may thus be sup- 
plied from the same converter. 

Development of the Electric Motor. — Oersted's dis- 
covery of electromagnetic action, in 1819, and the sub- 
sequent development of electromagnetism to which it 
gave rise, led to the invention of numerous machines 
designed to utilize electricity as a motive power by 



THE DYNAMO AND MOTOR. I9I 

means of the electromagnet. The principle of con- 
struction in all these machines consisted in energizing 
electromagnets by a battery current, and by their at- 
traction and repulsion producing mechanical motion, 
either rotary or oscillating. 

In the rotary motors a number of iron armatures, 
with or without inclosing coils, rotated in proximity to 
an equal number of stationary electromagnets; the 
rotation being produced by the attraction of each arma- 
ture in the same direction by the opposite magnetism 
of a stationary pole, and its repulsion by the similar 
magnetism of the pole from which it was receding; the 
polarity being reversed at the instant of closest prox- 
imity by a commutator fixed on the rotating shaft. 

The Jacobi motor was among the most noted of the 
coiled armature class, and the Froment and Neff motors 
of the naked armature class; the armatures in the Neff 
being stationary and the magnets rotary, while in the 
Froment the armatures rotated and the magnets were 
stationary. 

In the oscillating machines the armatures, consisting 
of a pair of loose fitting pistons, were attracted alter- 
nately into hollow electromagnets whose polarity was 
reversed by a commutator at the close of each oscilla- 
tion, and a reciprocating motion thus produced. These 
pistons and magnets were placed either vertically at the 
opposite ends of a horizontal walking-beam, as in the 
Gustin motor, or horizontally, end to end, as in the 
Du Moncel and Page motors, in which the oscillatory 
motion was changed to rotary by a crank. 

From 1830 to 1873 various motors of the above kinds 
were constructed, and attempts made to operate ma- 
chinery and propel boats and cars with them; one of the 
most noted of these experiments having been made by 
Jacobi with his motor at St. Petersburg in 1838; with 



192 DYNAMIC ELECTRICITY AND MAGNETISM. 

which he propelled a boat on the Neva, carrying 14 
passengers, at the rate of three miles an hour; employ- 
ing first a Daniell battery of 320 cells, and subsequently 
a Grove battery of 138 cells. The Daniell was objec- 
tionable on account of its great weight, and the Grove on 
account of its noxious fumes, while the rate of speed 
was far too low to be of any practical advantage. 

In 1851 Page propelled a car on the Washington and 
Baltimore Railroad, at a maximum speed of 19 miles an 
hour, with a i6-horse motor of his construction and a 
Grove battery of 100 cells. 

But the limited capacity of motors constructed as 
above, the cost and inconvenience of batteries of the 
requisite size, their want of constancy for such strong 
currents, and the risk and difficulty of their transporta- 
tion when filled with fluid and employed to propel cars, 
ivere fatal defects which could not be overcome, so that 
all such motors were found to be impracticable. 

In 1861 Pacinotti invented a motor, the armature of 
which has already been described; the whole construc- 
tion being practically the same as that of the Gramme 
dynamo which appeared subsequently. This motor, 
like all its predecessors, was energized by a battery, and 
hence could not be made practical, but was the same in 
principle as the improved motors now in common use; 
being simply a reversed dynamo in which an electro- 
magnetic current produced mechanical motion, instead 
of mechanical motion producing an electromagnetic 
current. 

Pacinotti recognized this principle of inversion, liav- 
ing found that by energizing his field-magnets by the 
battery current, or substituting permanent magnets for 
them, and rotating his armature mechanically, an elec- 
tric current was generated; so that the machine could 



■ THE DYNAMO AND MOTOR, I93 

generate motion by applying current, or current by ap- 
plying motion. 

The Dynamo as a Motor. — This principle of inversion in 
the dynamo, as discovered by Pacinotti, received its 
first practical application by Fontaine at the Vienna 
exposition in 1873; when he used a Gramme magneto- 
electric machine, attached to a pump, as a motor, put- 
ting it in operation by a current from a Gramme dyna- 
mo. This led to the discovery that the dynamo itself 
could be used as a motor and operated by a current 
supplied by another dynamo; thus substituting the 
stronger, cheaper, constant current generated by me- 
chanical power for the weak, dear, inconstant current 
generated by chemical action, and the superior energy 
of electromagnetic action for mere magnetic attraction 
and repulsion. 

Hence the motor and the dynamo are identical in 
principle and in construction, and the same machine may 
be used either as a generator or a motor. In practical 
use, however, the motor usually requires to be smailei 
and more compact, as the power generated by a steam- 
engine or a water-wheel can be converted into electricity 
most economically by a large dynamo, and distributed 
for running cars or operating light machinery by nu- 
merous small motors; a motor of a tew pounds weight 
having sufficient capacity to operate a sewing-machine 
or a small lathe. 

Principles of the Motor. — According to Lenz's law, re- 
ferred to in Chapter V, the reaction of an induced cur- 
'^rent, generated by the mechanical movement of a con- 
ductor, is always in opposition to the movement; hence 
the currents induced in the armature of a dynamo react 
in opposition to its rotary movement. This reaction is 
"the result of the potential difference generated by the 
movement, which, as has been shown, induces opposite 



194 DYNAMIC ELECTRICITY AND MAGNETISM. 

electromagnetic poles in the adjacent parts of .he arm- 
ature and field-magnets, producing attraction which 
tends to arrest the rotation. This attraction is electric 
as well as magnetic; the currents generated in the coils 
of opposite poles, facing each other, flowing in the same 
direction, and hence being mutually attracted. 

Now it is evident that a mechanical rotary force equal 
to this reaction is necessary to overcome it, and this 
constitutes almost the entire force required; the force 
requisite to overcome the friction and inertia of the 
armature being comparatively insignificant; and it is 
the rotation of the armature in opposition to this re- 
action which generates the current. 

If a dynamo, put in operation in this manner, be con- 
nected by conductors with another dynamo intended to 
act as a motor, the above conditions of potential differ- 
ence and reaction are produced in the second machine 
by the current from the first; and there being no me- 
chanical force in the motor to oppose this reaction, its 
armature rotates in the opposite direction to that of the 
generator, reproducing the mechanical force applied to 
the latter, less a certain percentage consumed in over- 
coming the resistance of the conductors. 

Hence the principle of the motor is simply that origi- 
nally discovered by Oersted, the rotation of a magnet 
by an electric current. 

But the motor thus operated generates a counter-cur- 
rent in opposition to that of the dynamo, and when the 
two machines are of equal capacity the opposing cur- 
rents vary as the relative speed of each machine; the 
motor current increasing and the dynamo current de- 
creasing till the speed is equalized, when the strength 
of the motor current becomes equal to that of the dyn- 
amo current less the amount necessary to overcome 



THE DYNAMO AND MOTOR. 195 

the motor's friction and inertia, and no effective current 
flows from the dynamo. 

This condition is soon attained when the machines are 
running without "load," that is, without doing useful 
work. But when the motor is made to operate machin- 
ery its speed is reduced in proportion to the load, and 
the counter-current decreasing as the speed, the current 
from the dynamo is increased in the same ratio. 

Hence, as the current varies inversely as the motor's 
speed and directly as the dynamo's speed, and the motor's 
speed varies inversely as the load, it follows that the 
speed of the dynamo must be made to vary directly as 
the load of the motor in order to maintain the requisite 
speed in the motor for the performance of useful work. 
Hence variation of load at the motor requires corre- 
sponding variation of power at the dynamo; the com- 
bined machines being simply an apparatus for the con- 
venient application of mechanical power to useful work; 
mechanical energy being transformed into electric 
energy by the dynamo, and this electric energy trans- 
formed into mechanical energy by the motor. 

Loss of Energy. — This double transformation entails 
a loss of about 15 ^ of the mechanical energy derived 
from the steam-engine or other source of power ; this 
percentage being spent in overcoming the friction, in- 
ertia, electric resistance, and self-induction of the ma- 
chines, including also their incidental waste. Besides this 
a loss is incurred in overcoming the electric resistance 
of the conductors, which varies in proportion to their 
cross-section and required length, and may equal an 
additional 10 per cent or more, according to the dis- 
tance to which the power is to be conveyed. A con- 
siderable loss is also often incurred by imperfect in- 
sulation, unavoidable under certain conditions, as in the 
running of street-cars. 



ig6 DYNAMIC ELECTRICITY AND MAGNETISM. 

Eddy Currents. — In both the dynamo and motor, cur- 
rents are induced in the iron core of the armature, un- 
less suppressed by specific means, which in the dynamo 
flow in the same direction as those induced in the coils, 
and in the motor, in the opposite direction. These cur- 
rents, regarded by Foucault as magnetic, are regarded 
by later writers as electric; a distinction which per- 
tains chiefly to their direction rather than their nature, 
if both kinds of energy be considered identical; and 
since they cannot combine with the currents in the coils, 
they serve no useful purpose; circulating as eddies in 
the iron, wasting energy and generating heat. 

The laminated structure of the armature core sup- 
presses them almost entirely in the dynamo, as has been 
shown, but it has been found more difficult to suppress 
them in the motor. For in the dynamo the two sets of 
currents, being in the same direction, tend to weaken 
each other, while in the motor, being in opposite direc. 
tions, they tend to strengthen each other, in accordance 
with the principles of current induction. Hence, in the 
dynamo the useful currents tend to suppress the eddy 
currents, and in the motor, to increase them. So that 
any eddy currents induced in the core of either arm- 
ature, notwithstanding the lamination, become more 
prominent in that of the motor. 

As these eddy currents are regarded as the chief cause 
of loss of energy in motors, the importance of suppress- 
ing them by complete lamination, with thin disks and 
perfect insulation, in motor armatures of all sizes, is 
apparent. 

Series, Shunt, and Compound Wound Motors. — The field- 
magnets of motors, like those of dynamos, are either 
series, shunt, or compound wound, and machines of 
€ach style are applied to the same work; practice being 
less definitely settled in motor work than in dynamo 



THE DYNAMO AND MOTOR. 197 

work, and opinion in regard to the adaptability of the 
different styles of winding to the different kinds of 
motor work varying. This arises from the complicated 
character of an apparatus composed of two machines 
having opposite functions and reversed modes of action, 
the adjustment of whose mutual relations, so as to 
adapt the apparatus to a varying external load, pre- 
sents a problem far more difficult of solution than the 
adaptation of a single machine to similar work, as in 
the dynamo. 

The shunt wound motor has the advantage of its in- 
ternal, automatic regulation, which adapts it to station- 
ary work having approximate constancy of load; while 
the series wound has been found better adapted to street- 
car work, where starting and stoppage, varying grade 
and speed, and varying number of passengers require 
manual regulation to adapt the current to this varying 
load on the motor, also prompt reversibility of motion 
and command of maximum energy at any rate of speed. 

In the Sprague motor, which is compound wound, 
differential regulation is obtained by opposing the shunt 
to the series current. 

In these different styles of winding the current enters, 
leaves, and circulates through the field in the reverse 
order to that in which it enters, leaves, and circulates 
through the corresponding styles of winding in the dyn- 
amo, except as changes are required in specific methods 
of regulation. 

Reversible Rotation. — But as reversal of mechanical 
motion is often desirable, motors are constructed in 
which the rotation of the armature is made reversible. 
This is accomplished in a very simple manner with two 
sets of brushes having opposite current connections; 
one set being lifted out of contact with the commutator 
by a lever attached to the brush-yoke, as the other set 



198 DYNAMIC ELECTRICITY AND MAGNETISM, 

is brought into contact; and the direction of the cur- 
rent through the armature being thus reversed, the ro- 
tation is reversed also. 

The Alternating Current Motor. — The construction of 
the direct current motor involves the double conversion 
of the electric current from alternating to direct in the 
dynamo and from direct to alternating in the motor, 
the armature currents in each machine being alternat- 
ing and the field-magnet and line currents direct. 

As this double conversion requires two commutators 
virith their wasteful resistance and sparking, various 
attempts have been made to eliminate it and produce 
a strictly alternating current motor; but previous to 
1888 such motors had not been made practically success- 
ful. The general introduction of the alternating current 
system at about that date created a special demand for 
them, and led to the construction of a practical motor 
of this kind by Nikola Tesla, based on the principle of 
the shifting of the magnetic poles in the dynamo, which 
has already been described. 

Tesla made the important discovery that if the field- 
magnets of a motor were made in the form of a ring, 
similar in construction to that of the Gramme armature, 
the coils on opposite sections of the core all around 
being separately connected in pairs, and the terminals 
of alternate, opposite pairs connected with two pairs of 
the usual collectors of the alternating current dynamo, 
on which the brushes make contact, the magnetic poles, 
induced in the motor by the transmitted currents, would 
be shifted continuously round the ring from pole to 
pole, making a complete revolution during each revolu- 
tion of the dynamo's armature, the polarity of each 
section of the ring being reversed by each alternation 
of current : and opposite poles being induced in the 
armature of the motor, it would be put in rotation in a 



THE DYNAMO AND MOTOR. 1 99 

corresponding manner by the resulting electromagnetic 
attraction and repulsion. 

The chief difference between this method of rotation 
and that of the direct current motor consists in the fact 
that, in the latter, the poles, having shifted to their 
relative positions, remain stationary, the armature ro- 
tating and its poles shifting continuously in the opposite 
direction as the armature rotates through them. Now 
if the field-magnets of the Tesla motor be regarded as a 
stationary armature, and the armature as a rotating 
field-magnet, we have practically the same relative con- 
ditions as in the direct current motor, with this differ- 
ence, that in the Tesla the poles rotate through a 
stationary armature, instead of the armature rotating 
through stationary poles; and the stationary poles of 
the rotating field-magnet maintain practically the same 
constancy of relative position to the rotating poles of 
the stationary armature as is found between the corre- 
sponding poles in the direct current motor. The relative 
conditions in the two motors are reversed but not essen- 
tially changed except by the elimination of the commu- 
tator from the Tesla. And the ability of the Tesla to 
start, stop, and maintain its rotation in synchronism 
with the dynamo is due to the reversed construction of 
the two machines, by which the magnetic poles are 
made rotary in the one where stationary in the other, 
and stationary in the one where rotary in the other. 

The Westinghouse Tesla Motor. — A Tesla motor, used 
in connection with the Westinghouse alternating current 
dynamo, is shown in Fig. 82, its construction being 
similar to that of the dynamo with certain modifications. 
The core of the field-magnets, or stationary armature, 
is laminated, each plate being circular externally, and 
having an even number of poles projecting inward. 
These plates, arranged symmetrically and properly 



200 DYNAMIC ELECTRICITY AND MAGNETISM. 



insulated, are bolted together between two caps which 
form the ends of the supporting frame; the lamination 
being shown on the outside in the cut, and twelve poles 
through the ventilating openings in the caps. The coils 
are in two separate series, wound oppositely on alternate 
poles; those of the same series being all wound in the 




Fig. 82. 
same direction, and those of the other series, which 
alternate with them, in the opposite direction. 

Each series has one of its terminals connected with 
the binding-post on the right, the other terminals being 
each connected with a separate binding-post on the 
left; the two left-hand posts being connected with 



THE DYNAMO AND MOTOR. 201 

separate collecting rings on the dynamo, and the right- 
hand post with a third ring; hence the current which 
enters by the right-hand post divides, producing oppo- 
site poles in the alternate, oppositely wound coils, and 
leaves by the two left-hand posts; while, at the next 
alternation, the current enters by the two left-hand 
posts, reverses the polarity, and leaves by the right-hand 
post. 

The armature, or rotating field-magnet, has the same 
construction as the armature of the Westinghouse dyn- 
amo, a single layer of copper wire covering the cir- 
cumference of a laminated iron cylinder, mounted on 
the shaft, the coils being looped round projections at 
the ends. The winding is continuous, as in the dynamo, 
but the two terminals are soldered together, so that the 
coils form a closed circuit without external connection. 
The insulation of the armature is not of essential im- 
portance, its E. M. F. being very low. 

In the direct current motor, with an armature rotat- 
ing between two pole-pieces, there are only four poles, 
two in the field-magnets and two in the armature on a 
line at right angles to them; but in this motor there are 
12 alternate poles in the field-magnets and 12 in the 
armature; each pair of armature poles, on opposite 
sides, being on a line at right angles to that joining a 
pair of field-magnet poles on opposite sides; the polarity 
of each pair being similar but opposite to that of the 
other pair. Hence the poles rotating round the station- 
ary field-magnets can never be more that -^^ of the cir- 
cumference in advance of the fixed poles which follow 
them; and there is a tangential pull between the dis- 
similar poles of each element, in opposite directions at 
opposite ends of each diameter, and a tangential push 
between the similar poles, producing the rotary force. 

As this attraction and repulsion varies inversely as 



202 DYNAMIC ELECTRICITY AND MAGNETISM. 

the square of the distance, and as the distance between 
poles varies with their number, it is evident that the 
rotary force must vary in the same proportion. But it 
should be distinctly borne in mind that this force is not 
due to magnetism alone, but is electromagnetic, as in 
the direct-current motor, the poles of each element 
being constantly deflected by those of the other element, 
in accordance with the principle discovered by Oersted. 

The mutual relations of speed and current, and. the 
generation of a counter current, explained in connection 
with direct current motors, apply also to this motor. As 
the speed, with a given load, varies in proportion to the 
rotary force, and the force varies with the number of 
poles, it is evident that the speed must vary as the num- 
ber of poles. Hence there should always be a sufficient 
number to insure the necessary speed for the required 
work, and this number may be greater or less than the 
number required in the connected dynamo; the relative 
speed of the two machines varying inversely as the 
relative number of poles in each. These general prin- 
ciples are, however, subject to various modifications 
dependent on special modifications of construction in 
each machine. 

The Tesla Motor as a Converter. — The construction of 
this motor is practically the same as that of the con- 
verter — a laminated iron case inclosing two insulated 
copper coils acting inductively on each other. This 
construction magnifies the inductive effect by bringing 
the coils of each element into close proximity, and 
especially increases the facility for rapid reversal of 
rotation, which is effected in the following manner: 

Reversal of Rotation. — If, while the motor is in opera- 
tion, the connections of the two left-hand binding-posts 
with the dynamo be reversed by the movement of a 
switch, the polarity will be reversed and the poles of the 



THE DYNAMO AND MOTOR. 203 

field-magnets made to rotate in the opposite direction. 
This reversal of polarity in opposition to the momentum 
of the armature changes the motor for an instant to a 
dynamo, generating a strong extra current in the arma- 
ture coils. This is the extra current of break, described 
in connection with the induction-coil as having superior 
strength, and, its direction being the same as if there 
had been no reversal, it opposes for an instant the 
reversed current from the dynamo, arresting the rota- 
tion of the armature, which, at the next instant, is 
reversed by the reversal of polarity; the successive 
steps following each other so rapidly that full speed in 
one direction is almost instantly changed to full speed 
in the opposite direction. 

Similar effects are produced in the reversal of the 
direct current motor, but the construction does not 
admit of equally effective induction. 

By passing the current through a special converter, 
the regulation of this motor can be adapted to any 
practical conditions required. 

Distribution of Power. — A number of motors may be 
operated on cars, or in shops, from one or more large 
dynamos, coupled together as generators, or employed 
separately, and centrally located; 'the parallel system of 
distribution, described in Chapter XI, being usually 
adopted, having been found more practical than the 
series system. 

This system is practically the same for distribution of 
power as for distribution of light, but in its application 
to cars special methods are required. One of the most 
common is to suspend a wire above the- track and make 
connection between it and the motor on each car by a 
trolley attached to the end of a connecting-rod which 
projects above the car. As the direct current is usually 
employed, it enters by this wire, insulated in the air, 



204 D YNAMIC ELECTRICITY AND MA GATE TISM. 

and returns by the rails, which are electrically connected 
together for this purpose; the motor having electric con- 
nection with them through the car axle and wheels, and 
the dynamo similar direct connection. 

As the resistance between the mains varies inversely 
as the number of cars employing current simulta- 
neously, the supply of current varies in like propor- 
tion; self-regulation being thus obtained, as in parallel 
lighting; any additional regulation required being sup- 
plied by resistance coils or otherwise. 

In cities w^here municipal regulations prohibit air 
lines, both conductors may be placed in a conduit; 
but as it is difficult, in such construction, to maintain 
proper insulation for naked wires with the open slot re- 
quired for connection with the motor, various ingenious 
methods have been devised to obtain the necessary in- 
sulation and connection. By placing the wires, properly 
insulated, in channels in the upper part of the conduit, 
on each side of the slot, they can be protected above 
and at the sides from dirt and wet; connection with the 
motor being made through the bottom of each channel; 
so that, with proper drainage, the insulation of the wires 
can be maintained; and by covering the motor connec- 
tions with insulating material they can also be protected 
from electric loss through contact with snow or mud. 

Elevated-Road Distribution. — On elevated roads the 
positive conductor can be connected with a central rail, 
both being properly insulated, and the track rails used 
for the return circuit, or any other convenient, econom- 
ical method adopted; insulation and protection being 
comparatively easy, presenting no such difficulties as 
surface roads. 

Thermo-Magnetic Motors. — The construction of these 
motors, which is still in the experimental stage, depends 
on the well-known principle that the heatings of a mag- 



THE DYNAMO AND MOTOR. ?.0^ 

net reduces its magnetism; hence if a rotary, iron arma- 
ture be mounted between the poles of a powerful elec- 
tromagnet and its opposite sections, at points unequally 
distant from each pole, be alternately heated and cooled, 
a constantly varying magnetic force is developed, and 
rotation produced and maintained; the amount of 
force, speed of rotation, and permanency and economy 
of the apparatus being dependent on the construction. 
The capacity of the experimental motors of this class, 
constructed by Tesla, Edison, Menges, and others, is 
quite limited. 

Such machines may also, like electric motors, be em- 
ployed as electric generators, by rotating the armature 
in opposition to the magnetic force. 

The economic value of a practical motor of this kind, 
by which the direct conversion of heat into power could 
be accomplished, would, for some purposes, be very 
great; since the present system of converting heat into 
power by the steam-engine, power into electricity by the 
dynamo, and electricity into power again by the motor, 
is wasteful and expensive, notwithstanding its many 
advantages: and if the elimination of the steam-engine 
and the dynamo could be accomplished, and the energy 
developed by the furnace utilized without loss by the 
substitution of a magnetic or electromagnetic engine 
for the steam-engine, a very desirable end would be at- 
tained. 

Still it is not probable that such an apparatus could 
supersede the use of the dynamo and motor as a con- 
venient, economical means for the distribution of power 
from a central station for running cars and many other 
purposes. 



20C DYNAMIC ELECTRICITY AND MAGNETISM, 



CHAPTER VIII. 
ELECTROLYSIS. 

We have seen that the electric development which 
takes place in a battery cell is proportional to the 
chemical reaction, and, conversely, it is found that the 
chemical reaction developed by an electric current de- 
rived from the cell, or otherwise, is proportional to the 
electric development. In explaining polarization it was 
shown how water may be decomposed by the electric 
current. This decomposition was discovered by Carlisle 
and Nicholson in 1800, and it was subsequently ascer- 
tained that many other chemical compounds could be 
decomposed in a similar manner. 

Nomenclature by Faraday. — Faraday, who made a very 
thoror.^h investigation of this subject, gave to this pro- 
cess the name of electrolysis, a term derived from Xvcj, 
to loosen, or separate, combined with tjXeKtpov^ and h^ 
called substances capable of such decomposition elec- 
trolytes; hence the term electrolytic is applied to the cell 
in which the process is conducted, and sometimes also 
to the products of the decomposition, to distinguish 
them from the same substance obtained by other means; 
as an "electrolytic" metal. 

The term electrode, which has already been defined, 
is also due to Faraday, and was first given to each of the 
wire terminals of the electric circuit connected with the 
electrolytic cell, though its use in other connections has 
since been found convenient, as already shown. He 
called the terminal by which the current enters the 



ELECTR OL YSIS. 20/ 

cell the anode, and that by which it leaves, the cathode; 
the former term being derived from ava oSoS, ascend- 
ing way, and the latter from Kara o6o5, descending 
way. 

He gave the name ions to the products of the decom- 
position, designating those which appear at the anode, 
or positive pole, as anions and those which appear at the 
cathode, or negative pole, as cations. Hence the anions 
are regarded as electronegative, and the cations as 
electropositive; each being attracted by the pole whose 
electric potential is supposed to be different from its 
own, and repelled by the one whose electric potential 
is supposed to be the same; electric energy overcoming 
chemical affinity. In the electrolysis of water, oxygen, 
appearing at the anode, is regarded as electronegative, 
and hydrogen, appearing at the cathode, is regarded as 
electropositive. 

Theory of Grotthuss. — The transfer of these atoms, or 
" migration of the ions," in opposite directions, which is 
a salient fact, is supposed to occur in accordance with a 
theory proposed by Grotthuss in 1805, and subsequently 
modified by Clausius. In every liquid a mutual inter- 
change of relationship is supposed to be constantly oc- 
curring among the molecules and atoms which compose 
them, producing motion in every conceivable direction, 
old groups being continually dissolved and similar new 
ones formed, and constancy of constitution thus main- 
tained under continual change. 

In Fig. 83 we have, in line i, an ideal view of this 
heterogeneous movement; each little oval representing 
a molecule of water, the two hydrogen atoms, which 
compose the hydrogen molecule, being shown by the 
shaded part and the oxygen atom by the unshaded 
part, an infinite number of such chains making up the 
mass of the liquid. In line 2, these molecules, under 



208 DYNAMIC ELECTRICITY AND MAGNETISM. 

the influence of an electric current from A to ^, are sup- 
posed to be reduced to a symmetrical phase, in which 
the oxygen part of each is turned towards the anode and 
the hydrogen part towards the cathode. Now a new 
grouping is supposed to take place, the oxygen of each 
molecule moving to the left to recombine wiai C.\& 
hydrogen of the adjoining left-hand molecule, and the 
hydrogen moving to the right to recombine with the 



I 



#'-- 
f^ 

i 



u: 



\\ 






Fig. 83. 



oxygen of the adjoining right-hand molecule; the new 
formation being represented by line 3, in which the 
oxygen part of the molecule at the left end of the chain 
and the hydrogen part of the molecule at the right end 
IS each left without a mate, and being prevented by 
electric action from combining with each other, each iS 
given off as gas. Thus while an interchange of atoms 
and molecules is taking place all along the infinite num- 
ber of lines of electric action throughout the entire 
mass, the opposite ions make their appearance only at 
the electrodes. The decomposition of every other eiec 
trolyte is supposed to take place in a similar manner. 



ELECTROLYSIS. ^OQ 

Electrolysis of "Water. — Different compounds vary 
greatly in their relations to electrolysis, and the elec- 
trolysis of the same compound often shows great varia- 
tion under different conditions. The feeblest current 
produces electrolysis in some cases, while in others the 
most powerful fails to produce it. Pure water, for in= 
stance, resists the strongest electrolytic action, while 
water slightly acidulated with sulphuric or chlorhydric 
acid is easily decomposed; the acid remaining appar- 
ently unchanged, while its presence reduces the electro- 
lytic resistance of the water. 

It has been suggested, in explanation of this, that 
there is a decomposition and recomposition of the acid, 
in this connection, in such a manner as to leave it un- 
changed; the decomposition of the water being indirect, 
through the agency of the acid; one or both gases be- 
ing derived from the acid, w^hich in turn receives from 
the water the sarpe amount of one or both which it has 
surrendered. In case sulphuric acid (SO^HJ is used, 
it could furnish both ; but in case chlorhydric acid 
(HCl) is used, it could furnish only hydrogen, while the 
hydrogen, taken from the water to replace this, would 
set free the proper combining proportion of oxygen. 
The theory given above shows how this may occur. 

It is evident that whether the decomposition of the 
water is direct or indirect, the final result would be just 
the same :he two gases being evolved in the exact pro- 
portions in which they recombine to form water. 

The high resistance of pure water to electrolysis does 
not absolutely prevent its decomposition. Gladstone 
and Tribe have effected it with zinc coated electrolyti- 
cally with spongy copper or spongy platinum, also with 
iron or lead similarly coated with copper: but, in this 
case, the electrodes being intimately connected, the re- 
sistance is reduced to a minimum, while decomposition 



2IO DYNAMIC ELECTRICITY AND MAGNETISM. 

ind the evolution of both gases with two platinum 
electrodes, separated, has not been found possible. But 
where the anode is an oxidizable metal, as copper, with 
which the oxygen, in the nascent state, can unite chemi- 
cally, the decomposition may be effected. This also 
occurs when sodium or potassium is brought into con- 
tact with water, the oxygen uniting with the metal and 
hydrogen being given off. 

Authorities differ in regard to the electrolysis of 
water under variation of pressure. It has been main- 
tained that electrolysis is influenced by pressure much 
in the same manner as evaporation is thus influenced; 
that under a pressure of 300 atmospheres — about 4500 
pounds to the square inch — even acidulated water can- 
not be decomposed, while in vacuo its decomposition 
may be effected by currents too weak to effect it under 
ordinary atmospheric pressure. But Bouvet claims to 
have effected it under a pressure of several hundred 
atmospheres, and to have found that the amount de- 
composed was independent of the pressure. 

As water is the usual solvent in solutions, its electroly- 
sis is usually inseparable from that of the substances 
held in solution, and becomes an important factor in the 
work required. 

Conditions of Electrolysis. — The required conditions of 
electrolysis are that the substance must be a liquid, 
either naturally or by liquefaction, a conductor of 
electricity, and a compound, one of whose constituents 
is usually a metal. Ice, though of the same chemical 
constitution as water, and a conductor of electricity, 
cannot be electrolyzed, because it is a solid. All the 
oils, and nearly all melted fats and resins, being non- 
conductors, are not subject to electrolysis : carbon 
bisulphide, the liquid chlorides of carbon, and many 
other substances belong to the same class. Solutions 



ELECTROL YSIS. 2 1 1 

of the salts of copper, silver, gold, potassium, and 
sodium are among the substances most easily electro- 
lyzed. 

The metallic elements usually appear at the cathode 
and are regarded as electropositive, and the non metal- 
lic at the anode and are therefore regarded as electro- 
negative. Hydrogen, which is considered a metal, 
appears, as we have seen, at the cathode. 

But in the liberation of the same element from differ- 
ent compounds, it may be either electropositive or 
electronegative according to the positive or negative 
character of its associate elements; positive and nega- 
tive expressing merely relative differences of potential 
under different conditions, and not absolute differences 
of physical constitution. 

Temperature has a very important influence; rise of 
temperature increasing both the electric conductivity 
and the electrolytic action. 

The time in which the action takes place is also of 
great importance; the results of rapid action generally 
differing considerably from those of slower action. Thus 
a simple metal, deposited at the cathode, may vary con- 
siderably in structure, or an alloy may differ in its com- 
position, according as the process of deposition is slow 
or rapid. 

Secondary Reaction. — Secondary reaction often occurs 
in electrolysis by which the liberated ions form new com- 
binations with each other or with the electrodes them- 
selves. We have had an instance of the former kind in 
the supposed decomposition and recomposition of acid 
in the electrolysis of acidulated water, and of the latter, 
in the union of the oxygen of water with potassium, 
sodium, or copper, used as the anode. 

Such secondary reaction may occur at either elec- 
trode or at both, with marked characteristics peculiar 



212 DYNAMIC ELECTRICITY AND MAGNETISM. 

to each. At the anode, the most common phenomena 
are corrosion of the anode, evolution of gas, and the 
adhesion of the ions to the anode, either as simples or 
new compounds; while, at the cathode, the ions, liber- 
ated either in a solid, liquid, or gaseous form, may 
either adhere to the cathode, be absorbed, dissolve, or 
escape. 

Alloys of the metals may also be formed at the 
cathode by the deposition of one metal upon another, 
also amalgams with mercury. 

Hence the permanent products of electrolysis may 
differ greatly from the elementary substances liberated, 
owing to the formation of new combinations during 
the process. 

The specific gravity of the liquid at each electrode 
often changes also, usually becoming heavier at the 
anode and lighter at the cr^^hode. 

Electrolysis of Mixed Coir pounds. — In the electrolysis 
of mixed compounds, the different elements are usually 
liberated in the order of their electropositive affinities; 
the least electropositive cation first, since it has the 
weakest chemical affinity, and the stronger ones subse- 
quently, in proportion to increase of current strength, 
or as reduction in the size of the electrodes increases the 
potential difference or E. M. F. 

But, by making the quantity of each substance in the 
solution proportionate to its electropositive strength, 
several elements may be liberated simultaneously; and, 
by increasing the proportion, the stronger may, in some 
instances, be liberated in larger amount than the weaker. 

By varying the proportions and other conditions in 
this manner, Favre was enabled to obtain from a mix- 
ture of the sulphates of cadmium, copper, and zinc, each 
metal separately, and also two, or all three, simultane- 
ously; and found that the various results depended on 



ELECTkOL YSIS. 2 1 3 

the energy of the battery, the electrolytic resistance of 
the salts, and the relative time of electrolytic action; 
and hence he concludes that, by thus varying the con- 
ditions, the different metals may be separated succes- 
sively from any mixture of metallic salts capable of 
electrolysis. 

Relations of Electrolysis to Heat. — The evolution of 
heat is a necessary result of all electrochemical work, 
and is due both to chemical action and to electric re- 
sistance. When elements combine chemically, as in the 
battery cell, heat is generated, and when the}^ are sepa- 
rated electrically, as in the electrolytic cell, heat is ab- 
sorbed; and the amount thus generated or absorbed 
bears a certain definite proportion to the work accom- 
plished and may be taken as its measure. 

This heat is distinct from that generated by the elec- 
tric resistance of the circuit, which varies in proportion 
to the amount of that resistance, and hence may be 
modified or controlled, while that due to chemical action 
is beyond control. 

In the battery cell there is always, in connection with 
the chemical reunion by which heat and current are 
generated, a certain amount of electrolysis by which 
heat and current are absorbed; and in the electrolytic 
cell there is always, in connection with the chemical 
separation by which heat and current are absorbed, a 
certain amount of chemical reunion by which heat and 
current are generated; and in both cells there is also 
the generation of heat by electric resistance. Hence 
when heat is absorbed, in either cell, there must be cor- 
responding electrolytic action, and, conversely, when 
such electrolytic action is developed there must be cor- 
responding absorption of heat. 

The electrochemical work required for the electroly- 
sis of any compound must be equal to that required to 



^14 DYNAMIC ELECTRICITY AND MAGNETISM, 

develop the amount of heat which would be generated 
by its chemical recombination, plus that required to 
overcome the electric resistance of the circuit. Hence 
the heat developed by electrochemical action in the 
battery is the measure of the electric work accomplished 
by the current, minus that expended in overcoming the 
electric resistance of the circuit; otherwise the results 
would not be in accordance with the law of the conser- 
vation of energy. 

Lowest Required Electromotive Force. — It has been 
shown that, in polarization, electrolytic action opposes 
electric generation; in like manner the action of the 
electrolytic cell opposes that of the battery, and when 
the opposing forces are equally balanced action in both 
must cease. Hence it is impossible to produce electroly- 
sis with a battery whose E. M. F. is only just equal to 
that of the electrolytic cell. 

If, for instance, the cell contain acidulated water, whose 
electrolytic reaction is 1.49 J volts, its electrolysis, with 
platinum electrodes, would be impossible with a current 
from a single Daniell cell, whose E. M. F. is only about 
I volt; hence two such cells would be the least number 
by which it could be effected, or a single cell having a 
higher E. M. F. than 1.49J volts, as a Grove or a Bunsen. 
The minimum E. M. F. required in each case varies 
with the nature of the compound to be electrolyzed, but 
it must always be in excess of that of the electrolytic 
cell, unless re-enforced by secondary action in that cell. 

When the anode is soluble and forms a new chemi- 
cal combination with the liberated anion the minimum 
E. M. F. required for the battery is greatly reduced. This 
is the case when a copper anode is used in the elec- 
trolysis of acidulated water; the chemical reaction, pro- 
ducing combination of the oxygen and copper, generates 
a current which re-enforces that of the battery, making 



ELEC TR OL YSIS. 2 1 5 

the electrolysis of even pure water possible, as already- 
shown. In this case the water may be decomposed by 
a single Daniell cell, or even one of less E. M. F. 

It is claimed that polarization does not occur with an 
anode of the same metal as that deposited on the cathode, 
and hence that a current of the lowest E. M. F. will 
produce electrolysis under these conditions. 

The opposing current set up in the electrolytic cell 
does not rise at once to its full E. M. F. Hence elec- 
trolysis may begin with a current of less E. M. F. than 
the required minimum, but cannot continue. This incipi- 
ent electrolysis has been attributed by Helmholtz to the 
presence, in the solution, of such uncombined atoms as, 
according to Clausius, have become separated from their 
former associates, but have not yet formed new combi- 
nations; hence their segregation can be effected by a 
current of less E. M. F. than that required to separate 
atoms already combined. 

Faraday's Laws. — The following laws were established 
experimentally by Faraday: 

1. The quantity of an ion liberated in a given time varies 
directly as the strength of the current, 

2. The zv eights of the different ions liberated from a series 
of different solutions by the same current in the same time vary 
directly as their chemical equivalents, 

3. Electrolysis is independent of the relative position of the 
electrolytic cell in the circuit. 

4. The number and amount of chemical equivalents which 
enter into combination in the battery are equal to the number 
and amount liberated by electrolysis in the circuit. 

It is immaterial from what electric source the current 
is derived. Faraday produced electrolysis even with 
the slight current from an electrostatic machine; and 
Sir Humphrey Davy, in 1807, separated the metals 
potassium and sodium from their bases, for the first 



2l6 DYNAMIC ELECTRICITY AND MAGNETISM. 

time, by the powerful current of a voltaic battery of 274 
cells. 

It is still an unsettled question whether an electric 
current can pass through a liquid without producing 
electrolysis. Observation seems to show that in some 
instances this may occur, and that in others the electro- 
lytic effect is small in proportion to the conductivity. 

Magnetic Effects. — Neither is it known to what extent 
magnetism influences electrolysis, as observation on this 
point has been very limited; but experiments by Rem- 
sen show certain marked peculiarities of manner in the 
deposition of copper, from its sulphate, under magnetic 
influence, which vary in proportion to the magnetic 
force, though the amount deposited remains unchanged. 

Peculiar magnetic effects have also been observed by 
S. P. Thompson in the deposition of lead. In 1826 
Nobili observed that the deposition of lead, from a solu- 
tion of its acetate, upon a platinum anode, occurred in 
the form of rings which gave rise to very beautiful 
chromatic effects, and are known as Nobilis rings. 
Thompson has found that when the deposition is made 
in a magnetic field, it ceases to have the circular form, 
and assumes a form. peculiar to the magnetic influence. 

Chemical Equivalence. — Faraday's second law may be 
illustrated as follows : There is in every molecule of 
water 2 atoms of hydrogen and i atom of oxygen, but 
each oxygen atom weighs 16 times as much as each 
hydrogen atom, hence the chemical equivalent of oxygen 
is 8, that of hydrogen being i ; its volume being only half 
that of hydrogen, though its atomic weight is 16 times 
as great : that is, a given volume of oxygen, as a cubic 
foot, weighs 16 times as much as the same volume of 
hydrogen, but there are 2 cubic feet of hydrogen in a 
given volume of water for every cubic foot of oxygen ; 



ELECTROLYSIS. 21/ 

and the liberation of these elements by electrolysis is 
therefore in this ratio. 

Hydrogen being the lightest of all known substances, 
its chemical equivalent is taken as the standard of com- 
parison for the chemical equivalents of all other sub- 
stances. The chemical equivalent of copper, for in- 
stance, is 311^, that being the weight of its atom as 
compared with that of 2 atoms of hydrogen ; and the 
chemical equivalent of silver is 108, that being the 
weight of its atom as compared with i atom of hydro- 
gen. 

Now let the same electric current be passed, for the 
same time, through three vessels, one containing acidu- 
lated water, another some salt of copper, as its sulphate, 
and the third some salt of silver, as its nitrate ; and, at 
the end of the time, let the products be weighed, and it 
will be found that for every gramme of hydrogen liber- 
ated there have been 313^ grammes of copper liberated, 
and 108 grammes of silver. 

Electrochemical Eq[uivalence. — The weight of any sub- 
stance liberated by a current of i ampere in i second is 
known as its electrochemical equivalent, and this is found 
to correspond practically with its chemical equivalent, in 
accordance with Faraday's law. Hence, if the chemical 
equivalent of any substance be multiplied by the electro- 
chemical equivalent of hydrogen, the product is the 
electrochemical equivalent of that substance. 

The electrochemical equivalent of hydrogen is found 
to be 0.000010352 of a gramme ; multiplying the chemical 
equivalent of copper by this, we get 31.7 X 0.000010352 
= 0.0003281584 of a gramme as the electrochemical 
equivalent of copper. In like manner the electro- 
chemical equivalent of silver is found to be 0.00T118016. 

Effect of Current Reversal. — Faraday's third law must 
not be understood as applying to the relative positions 



2l8 DYNAMIC ELECTRICITY AND MAGNETISM. 

of anode and cathode with reference to the direction of 
the current. If both are of the same substance and 
merely serve as conductors, as the platinum electrodes 
used in the electrolysis of water, their relative position 
is, of course, immaterial ; but if they are of different 
materials, one or both of which is soluble, reversal of 
relations by change of current or otherwise changes the 
results. Such reversal, during the process, removes the 
ions already deposited on the electrodes. Hence an 
alternating current is not adapted to electrolysis. 

Effect of Convection. — In a perfectly homogeneous solu- 
tion the strength of the current is the same in every part, 
and hence the liberation of the ions is uniform ; but the 
different parts of a solution are liable to a change of 
density during the process of electrolysis, producing 
differences in the liberation of the ions at different 
points on each electrode. This is especially the case 
when vertical electrodes are employed, with a metallic 
salt as the electrolyte. The specific gravity of the 
upper and lower parts of the solution, in proximity to 
each electrode, changes in consequence of difference of 
saturation ; increase of saturation occurring at the 
anode with descent of the more highly saturated portion 
of the electrolyte, and decrease of saturation at the cath- 
ode with ascent of the less saturated portion, which 
has been deprived in part of its metal. This convection 
produces difference of resistance, causing the main 
direction of the current to be from the upper part of the 
anode to the lower part of the cathode ; in consequence 
of which there is increased deposition of metal on the 
lower part of the cathode and a more rapid consumption 
of the upper part of the anode. 

This is more especially the case when a strong current 
is employed ; action being more uniform with a weak 
current. It is also more uniform with a horizontal posi- 



ELECTRO L YSIS. 2 1 9 

tion of the electrodes^ also with solutions of a viscous 
character, in which this convection occurs more slowly. 

Relative Conditions of Current and Electrolyte. — Ac- 
cording to Quincke, electrolysis is proportional to the 
strength of the current per unit of sectional area of the 
electrolyte, varies with the E. M. F., is inversely pro- 
portional to the distance between the electrodes, and is 
independent of the cross-section and conductivity of the 
electrolyte, when the resistance of the rest of the circuit 
is small in comparison 

The conditions of electrolysis thus far considered are 
those in which a current from an external source passes 
through an electrolytic cell, but it may also be effected, 
to a limited extent, by currents generated by contact 
between the electrodes and electrolyte, as follows : 
I. By dipping a metal into a solution consisting of a 
single liquidj as iron into a solution of copper sulphate 
or nitrate, which results in the deposition of a thin film 
of copper upon the iron ; the coppering of iron wire 
being done in this way. 2. By employing two solutions 
of different specific gravities, the lighter one resting on 
the surface of the heavier, and using, as in the former 
case, a single electrode in contact with both solutions. 
The current generated between the two liquids produces 
deposition on that part of that metal which, under the 
conditions, becomes the cathode ; the same result being 
produced by separating the solutions by a porous cell 
or partition^ and bending the metal so that its opposite 
ends dip into each solution. 3. By immersing, in a 
single solution, two metals, in contact externally, or con- 
nected by a conductor ; this produces a current from 
one metal to the other within the cell, causing elec- 
trolysis, the circuit being completed through the exter- 
nal connection. 4. By immersing two metals, connected 



220 DYNAMIC ELECTRICITY AND MAGNETISM. 

externally^ in two solutions separated by a porous cell 
or partition. 

These various methods will be recognized as instances 
of the partial electrolysis, already referred to, which oc- 
curs in every battery ; the cells being in fact various 
styles of battery cells. 

Electroplating. — The electrolytic deposition of a metal 
upon another metal is termed electroplatings and is the 
principal means by which plating is now accomplished; 
its first introduction as an art being by Richard Elking- 
ton of England, in 1840, though it had been performed 
experimentally by Wollaston in 1801, and by Brugna- 
telli in 1805. 

Various Details. — The principal metals used for this 
purpose are gold, silver, nickel, and copper, though other 
metals also are deposited in this way, as platinum and 
tin; also alloys, as brass, bronze, and german-silver. It 
is estimated that 125 tons of silver are used annually 
for electroplating in different parts of the world, 25 tons 
being thus used in Paris alone. 

A vat of suitable size is provided, usually made of 
pine, and lined with lead or gutta percha; enamelled 
cast=iron is also used for this purpose; and, for small 
establishments, vessels of glass, china, or stoneware are 
used. 

The current is furnished by a battery or other genera- 
tor, usually a dynamo in large establishments, and must 
be always maintained in the same relative direction. 

The solut-cn consists of some salt of the metal to be 
deposited, which yields it pure in sufficient quantity, and 
in the most economical and efficient manner; and the 
solvent is strictly pure water, usually distilled rain- 
water. 

The anode consists of one or more plates, usually of 
the same metal as that which is to be deposited, while 



ELECTROL YSIS. 



221 



the articles to be plated constitute the cathode; and 
both electrodes are suspended in the solution from cop- 
per bars which rest on copper strips, insulated from 
each other and connected respectively with the termi- 
nals of the generator, as shown in Fig. 84. 




Fig, 84. 

The Anodes. — The surface area of the nickel anode 
plates, used for nickel-plating, should equal or exceed 
that of the cathode surface, each cathode surface being 
exposed to an anode surface, and the depth of submer- 
sion of the anodes should be about two thirds the depth 
of the solution. For plating with gold and silver, less 
anode surface is required, and the exposure of both sur- 
faces of the cathode is less important, as the solutions 
part with their metal more easily than the nickel solution, 
and the anodes are more soluble. 

For nickel-plating plane, even surfaces, the distance 



222 DYNAMIC ELECTRICITY AND MAGNETISM, 

between the surface of anode and cathode should be 
aboi { 3 inches, but for surfaces having prominences or 
cavities the distance should be increased to 5, 6, or even 
10 inches, according to the amount of unevenness. 
For silver-plating, this distance should never be less 
than 4 inches. It is evident that while the distance, in 
either case, should not be such as to produce too great 
resistance, increase of distance tends to greater evenness 
of deposit on uneven surfaces, the ratio of difference in 
distance produced by such surfaces, as compared with 
the entire distance, being proportionally reduced by in- 
crease of distance, producing greater evenness of elec- 
trolytic action. 

Hooks of nickel wire are used for the suspension of 
nickel anodes, but copper wire may be used if it does 
not come in contact with the solution; and these hooks 
should be sufficiently numerous to insure full conduc- 
tivity. Silver anodes are suspended by iron wires or 
lead ribbons, and completely submerged, to prevent cor- 
rosion of the anode at the surface of the solution. 

As the metal of the anode, when the same as that de- 
posited, replaces that taken from the solution, its purity 
is a matter of great importance, affecting the color, 
brilliancy, and general quality of the work. But insolu- 
ble anodes of a different metal, or of some other sub- 
stance, are sometimes used with advantage. Platinum 
anodes are especially recommended for nickel-plating, 
being indestructible. But their exclusive use is not de- 
sirable, as the electrolytic resistance is greater with 
insoluble anodes, requiring increased expenditure of 
electric energy to overcome it; besides, the metallic con- 
stancy of the bath is continually changing^ being weak- 
ened by the abstraction of the metal, requiring repeated 
additions to maintain the requisite degree of saturation, 
Hence it is important that a certain proportion of the 



ELECTRO L YSIS. 223 

anodes, usually about one third, should be of nickel, 
both to maintain constancy and reduce resistance. 

Carbon anodes may be used where platinum is con- 
sidered too expensive. But even the best carbon anodes 
are liable to disintegration, and require to be renewed 
from time to time, which is a serious objection to their 
use. 

The beautiful deposits of green, red, and pink gold, 
seen on watchcases and jewelry, are obtained by the use 
of silver anodes for the green, and copper anodes for 
the red and pink, the operation being finished with a 
gold anode of the same color as the deposit. 

Plating Solutions. — The solution most generally used 
for nickel-plating consists of a double salt of nickel and 
ammonium, obtained by mixing in proper proportions, 
in distilled water, either nickel chloride with ammonium 
chloride, or nickel sulphate with ammonium sulphate. 
For certain purposes the character of the solution is 
modified by the addition of a little citric or chlorhydric 
acid. 

Solutions for silver-plating are composed variously as 
follows: silver nitrate; silver potassic cyanide; silver 
chloride and potassic cyanide; chlorides of silver and of 
soda; cyanides of silver and of potassium; silver nitrate, 
potassium carbonate, and ferricyanide of calcined po- 
tassium. 

The solution generally used for gold-plating consists 
of gold chloride combined with potassium cyanide; the 
chloride being obtained by dissolving the pure metal in 
a mixture of 2 parts chlorhydric and i part nitric acid, 
known as aqua regia. 

Auxiliary Operations. — The preparation of articles for 
plating and their subsequent finishing are among the 
most important parts of the process, requiring a series 
of operations which cannot be described here in full. 



224 DYNAMIC ELECTRICITY AND MAGNETISM. 

The principal preparatory steps are termed buffings 
cleansing, pickling^ and scouring. The buffing consists in 
polishing the surfaces by means of revolving disks and 
brushes, and finely powdered substances, as fine sand, 
pumice, emery, lime, and crocus, before plating, and in 
polishing nickeled surfaces in a similar manner after 
plating 

The cleansing, which is one of the first operations, is 
done by immersion of the article in hot potash or caustic 
soda; the pickling by its immersion m water acidulated 
with sulphuric acid; and the scouring by its immersion 
in a bath of nitric and sulphuric acids, to remove any re- 
maining traces of oxide. Special baths are also used 
with different metals for scourmg and other purposes. 

Zinc is given a light coating of copper before nickel- 
plating, by immersion in a solution of copper acetate, 
combined with salts of soda and potassium; this being 
necessary to procure adhesion of the nickel; and iron is 
sometimes similarly coated by immersion in a solution 
of copper sulphate and sulphuric acid. Zinc may also 
be amalgamated as a preparation for nickel-plating; and 
for silver-plating, amalgamation is a prerequisite for all 
metals, the bath for this purpose consisting of a solu- 
tion of mercuric binoxide in water acidulated with sul- 
phuric acid. 

Articles prepared in the above manner are suspended 
on hooks of suitable metal before immersion in the last 
preparatory bath, and must not be touched with the 
hand again before immersion in the plating bath, as 
the slightest contact of the bare hand, in nickel-plating 
especially, greases the surface sufficiently to produce an 
imperfect spot. 

The time of immersion in each preparatory bath 
varies from a few seconds to 15 minytes, the longest 
time being required for the potash bath, and the opera- 



ELECTROL VS/S. 225 

tions being accompanied with frequent rinsings; and, 
after plating, the articles are rinsed in water and died 
in hot sawdust before polishing or burnishing. 

Required Electric Energy. — The electric energy of 
E. M. F. and current strength required varies with the 
metal to be deposited. For nickel-plating especially it 
should be vigorous at the beginning and weaker towards 
the close, the E. M. F. varying from 5 volts to i volt; 
and when furnished by a battery, 3 Bunsen cells in 
series, or their equivalent, may be used at the beginning, 
and I Smee, or its equivalent, at the close. 

For silver-plating, an E. M. F. of not more than two or 
three volts is employed at the beginning, and a current 
strength of 50 amperes per square meter of cathode 
surface. 

For gold-plating the E. M. F. must not exceed one volt, 
as the solution has very low resistance, and the current 
strength must not exceed 10 amperes per square meter 
of cathode surface. 

Required Time of Immersion, and Thickness of Deposit. 
— The time of immersion in the plating solution varies 
with the metal to be deposited, with the metal to be 
plated, with the thickness of deposit required, and with 
the source of current employed. 

For nickel-plating, the time with a dynamo current 
varies from 15 minutes to an hour, and with a battery 
current from 2 to 5 hours. The average deposit is 
about 2 grammes per square decimeter, which gives a 
thickness of about -^^ of a millimeter. Heavier plating 
is liable to peel, unless special precautions are used; but 
the hardness of nickel renders heavy plating unneces- 
sary. 

The time for silver-plating varies from 3 to 4 hours 
with the dynamo current, and from 8 to 12 hours with 
the battery current. The average deposit does not ex- 



226 DYNAMIC ELECTRICITY AND MAGNETISM. 

ceed 3 grammes per square decimeter ; the average 
deposit on forks and similar-sized table-ware being 
from 80 to 100 grammes per dozen. 

Gold is deposited with great rapidity; the practical 
difficulty of the process being greatly increased by the 
necessity of rendering it immediately successful. A 
few minutes' immersion is sufficient to insure a good 
surface, which is usually very thin; gold-plating cover- 
ing more perfectly, and producing a better finish in 
proportion to thickness, than plating with other metals. 

Agitation of the Solution. — In all kinds of electro- 
plating frequent or constant agitation of the plating 
solution is important to insure the homogeneousness 
necessary for evenness of deposit; this agitation being 
sometimes maintained, in large establishments, by some 
special mechanical device. Special precautions are also 
necessary to insure good work on articles having deep 
cavities and sharp angles. 

Electrotyping. — The deposition of copper by elec- 
trolysis for the production of copies of woodcuts and 
similar engraved surfaces, and also of type, is known as 
electrotyphig^ and is the process by which the metal plates 
called electrotypes, used for all the finer grades of book 
and map printing, are prepared for the press. 

The details, which are comparatively simple, are 
briefly as follows: Impressions of .the type or cuts are 
taken with a press on plates composed of beeswax and 
graphite, each plate having sufficient surface for sev- 
eral such impressions. The surface is then shaved 
smooth and even, and " built up" to a thickness of about 
■jig- of an inch by additions of the melted composition to 
all the blank spaces; after which it is brushed with finely 
powdered graphite. It is then covered with a thin 
coating of copper, precipitated upon it from a solution 
of copper sulphate by iron filings; and the plates, thus 



ELE CTROL YSIS. 227 

prepared, are suspended by copper hooks in a solution 
of equal parts by weight of copper sulphate and sul- 
phuric acid in distilled water, at a distance of about 2 
inches from anodes of pure copper, of equal surface. 

With a dynamo-current, two hours' immersion gives a 
coating of the requisite thickness, but with a battery- 
current 12 to 14 hours is required. A single very large 
Smee cell furnishes a strong current of low E. M. F., 
about -f-^ of a volt, w^hich does not produce electrolytic 
resistance by decomposition of the water. 

The copper coating is then stripped from the plates, 
being loosened by expansion with hot water, its reverse 
surface coated with solder, and melted type-metal 
poured over it so as to produce a plate about \ of an 
inch thick. 

The different impressions are then cut from the large 
plates, straightened, planed smooth on the under sur- 
face, trimmed to symmetrical shape, and mounted, at 
the regular type height, on wood or metal bases. 

Electric Refining of Metals. — The refining of various 
metals by electrolysis has become an important art. It 
consists in obtaining them in certain required states of 
purity from the crude smelted products, and extracting, 
by the same process, such percentage of the precious 
metals as they may contain. 

Copper is one of the principal metals thus refined; 
the art having originated with Elkington, to whom pat- 
ents for the electric refining of this metal were issued, 
in England, in 1866. 

In a single copper refinery at Hamburg the average 
daily product, at a recent date, was 8 tons of refined 
copper, 2\ tons of which were chemically pure; and the 
gold extracted in a single year equalled \\ tons. 

The electricity in this refinery is generated with 
specially constructed dynamos, from which currents can 



228 DYNAMIC ELECTRICirY AND MAGNETISM. 

be obtained in parallel or in series, the parallel current 
from the largest dynamo having an E. M. F. of about 
4 volts and a strength of about 3000 amperes, and the 
series current an E. M. F. of about 8 volts and a strength 
of about 1500 amperes; the electric energy represented 
by the joint product of E. M. F. and current strength 
being the same in either case — 12,000 watts. 

Two series of 20 baths each are employed in connec- 
tion with this dynamo; each bath having an anode sur- 
face of 30 square meters, making a total of 1200 square 
deters ; the cathode surface being of equal amount, 
and the distance apart of the two surfaces from 2 to 4 
inches. The anodes are thick plates of the crude metal, 
and the cathodes, plates of the chemically pure metal, 
about I millimeter thick. The solution consists of cop- 
per sulphate, and the deposit occurs in thick layers, 
which are easily removed from the cathodes. The cop- 
per thus deposited liberates its combining equivalent of 
sulphuric acid, which unites with the copper of the 
anodes, furnishing a supply of sulphate by which the 
constancy of the bath is maintained. 

The precious metals are precipitated into the sedi- 
ment; from which they are separated, and subsequently 
refined by a separate process. 

Electric Reduction of Ores. — The separation of metals 
fr-om their ores is another important application of 
electrolysis. As such separation cannot be made suc- 
cessfully from crude ores, they must first be reduced 
chemically to salts capable of being electrolyzed, and 
the success of the process and its economy consists 
largely in the proper preparation of the ore in this man- 
ner; different salts of the same metal, treated by differ- 
ent methods, yielding to electrolysis with different 
degrees of facility, and producing the metal in varying 
degrees of purity and in variable quantity with the 



ELECTROLYSIS. 22g 

same current. Hence the nature of the preliminary 
process is often the sole condition of success or failure. 

Among the various ores reduced in this manner, the 
principal ones are those of zinc, lead, copper, silver, 
gold, aluminium, sodium, and magnesium. 

Sir Humphrey Davy, as we have seen, obtained po- 
tassium and sodium from potash and soda, experiment- 
ally, in 1807; but the first practical application of elec- 
trolysis to the reduction of ores was made by Bunsen in 
1854. He obtained aluminium, sodium, magnesium, 
barium, and other rare metals by this process in quan- 
tities comparatively large, operating on the chlorides of 
most of these metals. 

His process consisted chiefly in submitting the fused 
chlorides to electrolysis in a glazed porcelain crucible, 
maintained at a red heat, and divided into two com- 
partments by a porous earthenware partition reaching 
nearly to the bottom. By this method he electrolyzed 
aluminium and magnesium with a current of 15 to 20 
volts, derived from a Bunsen battery, using electrodes of 
coke carbon, the metals going to the cathode and the 
chlorine to the anode. 

H. E. S. C. Deville subsequently improved Bunsen's 
process for the reduction of aluminium. He mixed 
2 parts by weight of aluminium chloride with i part 
common salt (sodium chloride) pulverized, fused the 
mixture at a temperature of 218° C, and electrolyzed it 
in a glazed porcelain crucible, maintained at a temper- 
ature of 183° C. He used a cylinder of charcoal for the 
anode, immersed in a portion of the fused mixture con- 
tained in a porous cell placed in the crucible ; salt being 
added in this cell to fix the chloride and prevent its 
volatilization. 

The cathode was a platinum plate, and two battery 
cells furnished sufficient electric energy, the resistance 



230 DYNAMIC ELECTRICITY AND MAGNETISM. 

being very low. The deposit contained a percentage of 
common salt, which was subsequently removed by dis- 
solving it in water ; and the metal was further purified 
by successive fusions, and treatment with the double 
chloride of sodium and aluminium as a flux. 

About 1885 C. E. Becquerel separated silver, copper, 
and lead from their ores by electrolysis ; the silver ore 
being first reduced to a chloride and the copper and 
lead ores to sulphates. 

Instead of a current derived from an external battery, 
he used the electrolytic cell as his battery, the liquid 
being a solution of the ore, while the electrodes were 
composed of zinc, iron, or lead for the anodes, and cop- 
per, tin, or carbon for the cathodes ; grouping the cells 
as required for E. M. F. or current strength. 

For copper he arranged them as gravity cells, a light 
solution of iron sulphate being superposed on a denser 
solution of copper sulphate, a cast-iron anode being 
placed in the iron solution, and a copper cathode in the 
copper solution, the deposit being made on the cathode. 

The production of electric energy at economical rates 
by the dynamo has revolutionized these earlier methods 
of the electrolytic reduction of ores by battery currents 
which were too expensive to be practical ; and their chief 
value now consists in indicating the nature of the re- 
quired preliminary preparation. But even at the pres- 
ent comparatively low cost of electric energy, the electric 
process is not always the most practical or economical, 
and its application in any particular case must be de- 
termined by the attendant circumstances. Where water- 
power is cheap and abundant, and fuel expensive and 
scarce, its application is likely to be more practicable 
than where these conditions are reversed ; power being 
easily convertible into electric energy, while the fuel 



ELECTROLYSIS, 23 1 

required for the smelting process might make it more 
expensive than the electric process. 

The chlorides are still the salts most generally em^ 
ployed, though the sulphates, nitrates, and acetates are 
preferable for some metals and for some processes. 
These salts are prepared from the ores by roasting, 
fusing, pulverizing, mixing with various substances, 
treating with acids, and other operations, according to 
the nature of the ore or the process ; and are then re- 
duced to the liquid condition for electrolysis, either by 
fusion or by solution in water, the nature of the ore or 
process, as before, determining the method required. 

The Hall Process for Aluminium. — The process of C. 
M. Hall of Oberlin for the electric reduction of alumin- 
ium from its ores, patented in 1889, has been put into 
successful operation by the Pittsburg Reduction Com- 
pany, resulting in the production of the metal, nearly 
pure, in large quantities and at a greatly reduced price. 

It is substantially as follows : 

A steel crucible lined with carbon contains a bath, 
lighter and more electropositive than aluminium, com- 
posed, by weight, of 234 parts calcium fluoride, or fluor- 
spar ; 421 parts of the double fluoride of aluminium and 
sodium or cryolite ; 845 parts aluminium fluoride, ob- 
tained by saturating hydrated alumina, A^HOe, with 
hydrofluoric acid. The bath's chemical composition is 
represented approximately by the formula NaaAlaFg + 
CaAUFe ; to which is added 3 or 4 per cent of calcium 
chloride, CaClu, to prevent the abnormal increase of elec- 
tric resistance, otherwise liable to occur from the for- 
mation of certain impurities. 

The crucible being set in a furnace, the bath is fused 
at a red heat, and alumina in the form known as bauxite, 
an anhydrous oxide of aluminium, or the pure anhydrous 
oxide, AliOa, artificially prepared, is dissolved in this 



232 DYNAMIC ELECTRICITY AND MAGNETISM. 

fused bath and subjected to electrolysis with a dynamo 
current of 4 to 8 volts E. M. F.j which is sufficient to 
electrolyze the alumina, but not the bath. 

Carbon electrodes are employed, the anode being im- 
mersed in the bath and the carbon lining of the crucible 
forming the cathode. The aluminium goes to the ca- 
thode, sinking to the bottom on account of its greater 
specific gravity, where it can easily be drawn off; and the 
oxygen goes to the anode, where it unites with the car- 
bon, forming carbonic acid, CO2, which escapes as gas; 
the anode being thus consumed at the rate of about 
I pound of carbon to i pound of aluminium produced, 
and requiring frequent renewal. The presence of the 
calcium chloride prevents more rapid carbon consump- 
tion by suppressing the formation of carbonic oxide, CO, 
otherwise liable to occur, and which consumes double 
the amount of carbon, the oxygen uniting with the car- 
bon in the proportion of i to i instead of 2 to i, as 
shown. 

The electrolysis proceeds continuously, aluminium 
being drawn off and alumina added in sufficient quan- 
tity to keep the bath saturated with it, though an ex- 
cess is not injurious, as it merely sinks temporarily and 
is subsequently taken up. The bath also requires occa- 
sional additions of material to renew the loss due to 
volatilization and other causes; the calcium chloride 
volatilizing most rapidly, and its abnormal reduction 
being indicated by a fall of current due to increase of 
electric resistance. 



ELECTRIC STORAGE, 233 



CHAPTER IX. 
ELECTRIC STORAGE. 

The Leyden Jar and Condenser. — The storage of elec- 
tric energy in the Leyden jar was one of the earliest 
discoveries in electric science. A full description of 
this instrument and its principles is given in the author's 
" Elements of Static Electricity," so that it is only neces- 
sary here to say, that it is simply a glass vessel, coated 
with metal on both surfaces to within a few inches of 
the top, which is left bare for insulation. 

An electrostatic charge, positive or negative, given to 
either coating, usually the inner, by a static machine 
or spark coil, produces by induction a charge of oppo- 
site potential on the other coating, when connected with 
the earth or opposite pole of the machine or coil, and 
the two coatings remain in this electric condition till 
gradually restored to the same potential by the slow 
convection of the air; but an instantaneous discharge 
may be produced, attended with spark and snap, by 
making a connection between them by a conductor. 
And it is characteristic of this instrument, that while 
the charge is received gradually, occupying usually 
some seconds or minutes, the discharge, produced as 
above, is always instantaneous, and nearly complete. 

The condenser, described in connection with the in- 
duction coil, is an instrument of the same character, re- 
ceiving and surrendering its charge in a somewhat sim- 
ilar manner. 

Grove's Gas Battery. — Polarization is another instance 
of electric storage, and observation of this phenomenon, 



234 DYNAMIC ELECTRICITY AND MAGNETISM, 

and of the fact that the oxidation of the copper anode 
in an electrolytic cell produces similar storage, led to 
some experimental investigation of the subject by Gau- 
therot and Ritter early in the present century. In 1842 




Fig. 85. 

Grove constructed a gas battery on this principle, which 
is illustrated by Fig. 85. 

A three-necked flask, F, contains acidulated water 
into which are inserted two inverted tubes, containing 
respectively oxygen and hydrogen, designated by O and 



ELECTRIC STORAGE. 235 

H. Platinum wires, sealed into the upper ends of these 
tubes, are connected with platinum electrodes in contact 
with the gases above and water below, and the external 
circuit is completed through copper conductors whose 
terminals dip into mercury cups. 

When the circuit is closed the gases recombine to 
form water, generating an electric current, which in the 
cell is from hydrogen to oxygen, and externally from 
oxygen to hydrogen, and whose E. M. F. is equal to that 
required to electrolyze water, 1.49I- volts. 

It is evident that the gases could either be evolved 
by a separate chemical process and admitted to the 
tubes previous to the latter being connected with the 
cell, or be evolved directly from the acidulated water 
of the cell by a battery or dynamo current. In the lat- 
ter case the total amount of electric energy generated 
by the recomposition would equal that expended in the 
decomposition, and in the former the amount of electric 
energy obtained would be in the same proportion for 
the same amount of gas recomposed. In either case 
there is storage of electric energy by cheifitcal decomposition, 
7vhich is recovered by chemical recomposition; and this is 
the principle of chemical electric storage as developed 
in the various styles of the apparatus, known as the 
storage battery, accumulator, or secondary cell. The gener- 
ation of electric energy must always follow the recom- 
bination, whether the elements are evolved in the gas- 
eous form by insoluble electrodes like platinum, or in 
the solid form by combination with soluble electrodes, 
of which the oxide formed on a copper anode in the 
electrolysis of water is an instance. 

Grove constructed similar batteries with other gases, 
and also with plates covered with metallic peroxides. 

Wheatstone, Siemens, and Kirchoff made similar ex- 
periments, but Gaston Plante's discovery, in 1859, of 



236 DYNAMIC ELECTRICITY AND MAGNETISM, 

the special adaptation of lead plates for this purpose, 
opened the way for the practical success of electric 
storage. 

Plant6's Secondary Cell. — Plante constructed a cell, 
using as electrodes two large sheets of lead rolled to- 
gether and electrically insulated from each other with 
strips of gutta-percha, as shown in Fig. 86; the method 




Fig. 86. 



of rolling being shown at A, and the sheets, rolled and 
clamped, at B, projecting strips of lead being left at- 
tached to each for terminals. They were then im- 
mersed in water acidulated with ten percent sulphuric 
acid, in a tall glass jar, and subjected to the action of a 
battery current supplied by two or more cells. A por- 
tion of the water being decomposed, the oxygen evolved 
at the anode combined with the lead, forming a dioxide, 
and the hydrogen was occluded on the cathode. 

When the anode ceased to absorb oxygen, as indicated 
by the escape of the gas, the cell was disconnected from 
the battery, and discharged by making an external con- 
nection between the terminals of the electrodes, and 
then recharged with a reversed current. 

This process was repeated during a period of several 



ELECTRIC STORAGE. 237 

months, the time of charging being continually increased 
from a few minutes at first to several hours subse- 
quently, with long and increasing intervals of repose 
previous to each discharge and reversal; its object being 
to cover one of the plates with a thick coating of dioxide 
of lead, and the other with a coating of spongy lead of 
equal thickness. 

Chemical Reaction. — The chemical reactions, as de- 
scribed by Gladstone and Tribe, are substantially as 
follows: The first charging produces only a thin film 
of the dioxide on the anode and of the hydrogen on 
the cathode; but the discharging changes the dioxide, 
Pb02, which is insoluble in sulphuric acid, to monoxide, 
PbO, which is at once reduced to sulphate, PbSO^, by 
the acid of the solution; the liberated oxygen atom 
uniting with the lead of the cathode and forming mon- 
oxide, which is also reduced to sulphate, as on the 
anode; the result of the discharge being a thin film of 
lead sulphate on each plate. 

During the second charging the sulphate of the plate, 
now made the anode by reversal of current, is decom- 
posed, the sulphuric acid, absorbed to form the sul- 
phate during the repose and subsequent discharge, is 
restored to the solution, and the lead, thus liberated, 
combines with the oxygen liberated simultaneously from 
the water and forms the dioxide. The hydrogen, also 
liberated from the water, goes to the plate now made 
the cathode and decomposes its sulphate; restoring the 
sulphuric acid to the solution, and liberating the lead, 
which adheres to the plate as a spongy coating. 

The respective results of each subsequent charging 
and discharging are the same as those just described; 
and as the spongy lead affords increased interior sur- 
face, the chemical reactions and formation of dioxide 
are proportionally increased. 



238 DYNAMIC ELECTRICITY AND MAGNETISM. 

But increased thickness of the dioxide produces in- 
creased resistance to chemical reaction; hence arises 
the necessity for the period of repose before dischargin^lf, 
during which the chemical reaction of the anode plate, 
by the strong affinity of the lead for oxygen, changes 
some of the dioxide to monoxide, which the acid im- 
mediately changes to sulphate, and thus the resistance 
is lessened. 

There is also a resistance arising from an interior 
coating of sulphate, not reduced to dioxide by the 
charging, which remains in immediate contact with the 
plates and impedes the local action of repose, making 
longer periods of repose necessary as the coatings in- 
crease in thickness. 

Hence the electric formation of the plates consists of 
three distinct processes, the charging for the formation 
of dioxide on the one and spongy lead on the other, the 
repose for local action, and the discharging for the pro- 
duction of sulphate on both; the plates when completed 
consisting respectively of lead dioxide and spongy lead 
adhering to interior supports of sheet lead; and sub- 
sequent charging, for practical use, is always in the same 
direction, alternation being discontinued. 

The charge may be given either by a battery or a 
dynamo, usually the latter; the chemical reactions, 
when the cell is in practical use, being just the same as 
during the preparatory process; the electric effect being 
the absorption of electric energy by the conversion of 
sulphate into dioxide during the charging, which is re- 
covered by the conversion of dioxide into sulphate dur- 
ing the discharge. 

It has been observed that often when a partially dis- 
charged cell is given a short period of repose, the sub- 
sequent discharge shows increased electric energy. 
This is accounted for on the hypothesis that when the 



ELECTRIC STORAGE, 239 

discharge is rapid some of the sulphate, formed on the 
anode from the dioxide, is reconverted into dioxide by 
the excess of oxygen developed, producing a propor- 
tional reduction of potential difference between the 
plates; but that during the short repose this dioxide is 
again reduced to sulphate and the potential difference 
restored. 

The maximum E. M. F. of the Plante cell is about 
2.54 volts. By means of a commutator of special con- 
struction, Plante could instantly join a battery of 20 
such cells either in series or in parallel. He used the 
parallel connection for charging, which he accomplished 
with 2 Bunsen cells, the resistance, with this connec- 
tion, being very low, and the series connection for dis- 
charging, by which he obtained a maximum current 
equal to that of 30 large Bunsen cells. 

The duration of the discharge depends on the resist- 
ance of the external circuit, varying from a few minutes 
to several hours according to the amount o.f current re- 
quired; and it ceases when the dioxide is all changed to 
sulphate, but should be terminated sooner to prevent 
injury to the plates from the excessive formation of sul- 
phate. 

The Faure Cell. — The tedious, expensive process re- 
quisite for the electric formation of the Plante plates 
led to the construction by Camille A. Faure, about 
1880, of plates prepared by covering sheet lead with a 
paste made of red lead and sulphuric acid; the coating 
being confined to the surface by a covering of paper 
and by felt placed between the plates, which also served 
the purpose of insulation. 

Thus prepared and rolled together, they were placed 
in a glass jar, in water acidulated with sulphuric acid, 
and the coating subjected to electrolysis with alternation 



240 DYNAMIC ELECTRICITY AND MAGNETISM. 

of current, by which the red lead, known also as minium^ 
PbgOi, was changed in a few days to lead dioxide and 
spongy lead, on each plate respectively, and the cell was 
ready (or practical use. 

Chemical Reaction. — ^.The chemical reaction, according 
to Gladstone and Tribe, is substantially as follows: On 
the immersion of the plates in the acidulated water, 
there is, at first, a purely chemical reaction, by which 
the minium on both plates is gradually changed, from 
the surface inwards, into a mixture of the dioxide and 
sulphate of lead, with evolution of water, thus, PbgO^ + 
2H2S04=Pb02-f 2PbS04+2H20. But oxygen and 
hydrogen being liberated by the electric current, the 
oxygen goes to the anode and changes the sulphate 
into dioxide and sulphuric acid, thus, 2PbS04 + 2H2O + 
O2 = 2Pb02 -f- 2H2SO4. The hydrogen goes to the ca- 
thode, changing the dioxide to spongy lead, with evolu- 
tion of water, thus, Pb02 + H^ = Pb + 2H2O; and chang- 
ing the sulphate to spongy lead and sulphuric acid 
thus, 2PbS04 + H^ = 2Pb 4-2H2SO4; reversal of current 
being necessary to electrolyze the heavy coatings com- 
pletely. 

The chemical reaction of the discharge is the for- 
mation of lead sulphate on both plates, and that of sub- 
sequent charging the reconversion of this substance to 
dioxide and spongy lead, as before. 

Defects of the Faure Cell. — While the Faure cell could 
be produced much more economically than the Plante, 
and was equal to it in electric energy, it had many 
serious defects which proved fatal to its practical suc- 
cess. The felt, preventing the free circulation of the 
fluid, seriously impeded electrolysis; it soon became 
corroded by the acid and partly removed in patches, 
and ceased to insulate. The coating failed to adhere 



ELECTRIC STORAGE, 



241 



properly, sloughing off and falling to the bottom. 
Hence in a short time the cell became worthless. But its 
invention demonstrated the possibility of practical suc- 
cess by some similar method of construction, to ascer- 
tain which the investigations of Brush, Swan, Sellon, 
Volckmar, and others were immediately directed. 

Improved Faure Cell. — The result of these investiga- 
tions was the production, about 1886, of an improved 
cell, the principal feature of which is the improved style 
of plate illustrated by Fig. 87, which consists of a lead 
grid, shown at A^ having its openings wider at the sur- 



vs 



U 

n 



Z/ 



A 

Fig. 87. 



faces than in the interior, as shown by the enlarged sec- 
tion at B. These openings are filled with a paste made 
of minium and sulphuric acid for the positive plates, and 
of litharge and sulphuric acid for the negative; litharge 
being a red lead monoxide, PbO, more easily reduced 
than minium, for which reason it is preferred for the 



242 DYNAA/^C ELECTRICITY AND MAGNETISM, 

negatives, to facilitate the reduction of the paste to 
spongy lead, which is more difficult than its reduction 
to lead dioxide on the positive plates. 

The advantages of this style of plate are that it gives 
a firm support to the paste, the plugs being held in the 
grids by the form given them by the openings, which 
obviates the. necessity for the intervening felt and paper, 
allowing free circulation of the fluid and more perfect 
electrolysis. 

The cells are made of different sizes, stationary 
and portable; the stationary cells having glass vessels, 
and the portable, hard-rubber vessels. The 23-plate 
stationary cell, shown in Fig. 88, has 11 positive plates 
and 12 negative; each set attached to a lead cross-bar 
above and at the center, by which the plates are held at 
a fixed distance apart; the two sets interlocking, so that 
positive and negative plates alternate and are insulated 
from each other by two rows of hard-rubber forks. 
Each plate is \ of an inch thick, and the space between 
adjacent, positive and negative plates, -^ of an inch 
wide; and the two outside, negative surfaces being in- 
active, each set has 22 interior, active surfaces. 

A thick plate of glass, under the central cross-bar and 
plate connections on each side, supports the plates, so 
as to leave a space underneath for the free circulation of 
the fluid; each set being supported, on its opposite side, 
by plate projections which rest on an insulating hard- 
rubber strip above each cross-bar as shown; two stout 
rubber bands holding these supporting plates and the 
lower parts of the lead plates in position. A lead bar, 
projecting from the cross-bar of each set, can be bent 
into any convenient position for making connection with 
adjoining cells. 

These plates are immersed in water acidulated with 



ELECTRIC STORAGE, 



243 



sulphuric acid, contained in a glass jar loj inches 
long, 8J inches wide, and 9I inches high ; the entire 
weight of jar and contents being 50 lbs. 




The glass jar has the advantage of allowing inspec- 
tion of the interior without disturbing the contents, by 
which the condition of the plates may be observed, and 



^44 DYNAMIC ELECTRICITY AND MAGNETISM. 

short-circuiting from the buckling of plates or the lodg- 
ing of loose paste plugs between them remedied; but 
its comparative frailty and weight are objections to its 
use for the portable cells required on cars and else- 
where. Hence a portable cell of the same capacity and 
number of plates is constructed with a covered, hard- 
rubber jar, made shorter below than above so as to 
furnish supporting ledges for the plates at the opposite 
ends, which take the place of the glass supporting plates 
employed in the stationary cell. The weight of this 
cell is 40 lbs., its height about the same as that of fclie 
.stationary cell, and its other dimensions about one 
fr»urth less. 

The 15-plate stationary cell has 7 positive plates, each 
•3^2- of an inch thick, and 8 negatives, each -j-^ of an inch 
thick, contained in a glass jar lof inches long, \2\ inches 
wide, and i3f inches high; the entire weight being 130 
lbs., and the storage capacity 300 ampere-hours, double 
that of the 23-plate cell. The supporting plates are of 
!iard rubber, with openings for inspection, and are each 
held in position by two metal rods which pass through 
loops in the positives at one end and in the negatives 
at the other, binding the plates of each set together 
oelow^ and furnishing electric connection between them. 

Electric Preparation of the Plates. — Each set of plates, 
positive and negative, is electrolyzed separately before 
they are combined in the cell intended for practical use; 
special sets of temporary plates, or dummies, of each 
kind being used for this purpose, which makes the old 
process by reversal of current unnecessary. The nega- 
tives, although thinner than the positives, require six 
days for the reduction of the litharge to spongy lead, 
while the minium of the positives is reduced to diox- 
ide in 24 hours. 

Electric Energy of Improved Cell. — The E. M. F. of this 



ELECTRIC STORAGE, 245 

cell is about 2 volts, and its internal resistance .001 to 
.005 of an ohm. Its current, as in the Plante, depends 
on the external resistance; 30 amperes for jo hours 
being considered an economical working rate. for the 
large 15-plate cell. If less current is required the time 
of discharge becomes proportionally longer; and a cur- 
rent of 300 amperes may be obtained for an hour, but 
such rapid discharge is injurious to the cell. 

Effects of Charge and Discharge on the Plates. — As both 
the charge and discharge result in different forms of 
chemical reaclion, it is obvious that ample time should 
be allowed for this reaction to produce the required 
chemical changes; the normal rate under varying con- 
ditions being ascertained better by practical experience 
than by arbitrary rule. 

Charging is always accompanied by the evolution of 
gas, which, as has been shown, is chiefly absorbed, 
while a certain percentage escapes; hence if the rate of 
charging is excessive there is an abnormal escape of gas 
and useless consumption of current: there is also an 
abnormal development of heat, which may result in de- 
struction of the plates. 

As there can be no further absorption of gas when 
the chemical reaction of charging is completed, its ab- 
normal escape with a normal current indicates an over- 
charge, resulting as before in waste of current, but not 
in injury to the plates. 

But as the chemical reaction of the discharge results 
in the absorption of sulphuric acid and the formation 
of lead sulphate, a hard unyielding substance, in con- 
siderable quantity on both plates, and in excess of the 
material which it replaces, due to the absorption of 
acid, it is evident that if the action is too rapid, the 
plugs on the surfaces most exposed to chemical and 
electric action will become sulphated to a greater degree 



246 DYNAMIC ELECTRICITY AND MAGNETISM. 

than on the opposite surfaces, producing unequal ex- 
pansion, with warping, or buckling of the plates, as a 
result; the same result also occurring from an excessive 
formation of sulphate if the discharge is continued too 
long. 

E. M. F. of Discharge.— The E. M. F., during the first 
half-hour of discharge, is about 2.25 volts, being slightly 
increased by the supplementary reaction of a small 
amount of gas which adheres to the plates after charg- 
ing; it then drops to about 2.4 volts, remaining nearly 
constant, with a slight decline to 2 volts or less, till the 
dioxide is mostly reduced to sulphate, when it begins 
to decline rapidly; which indicates that the discharge 
should cease to prevent injury to the plates. 

Conductivity and Buckling. — It is important that there 
should be a sufficient supply of sulphuric acid present 
to maintain the requisite conductivity during the dis- 
charge, when it is rapidly absorbed to form the sulphate; 
otherwise the fluid will soon be deprived of its normal 
quantity and the resistance abnormally increased. An 
excess of the acid is also injurious, causing the sulphate 
to form too rapidly, with buckling of the plates as a 
result. Such excess is liable to occur in the lower part 
of the cell, where the acid, from its greater specific 
gravity, accumulates, causing a corresponding reduction 
in the upper part. This increases the conductivity and 
chemical and electric action below, with corresponding 
decrease above. Hence buckling usually increases 
downward. 

Buckling, if not excessive, and if in the same direc- 
tion o-n all the plates, does not interfere with the action 
of the cell. But it always tends to loosen the plugs, so 
that they are liable to drop out and fall into the space 
between the plates, producing a short circuit. There is 
also liability to short-circuiting by contact between 



ELECTRIC STORAGE. 247 

positive and negative plates, if the buckling is in oppo- 
site directions. 

Weight of Cells. — When a storage battery is used on a 
car to furnish light or motive power, reduction of weight 
becomes highly important, as a large number of cells 
are usually required, and their aggregate weight will 
often equal 3500 pounds. 

Composition of Grids. — Various metals and alloys have 
been tried for grids, but lead still has the preference, 
and is in general use. The addition of a small per- 
centage of antimony, as a flux, aids in producing more 
perfect castings; lead alone failing to flow into the nar- 
row spaces in the molds with the requisite facility. The 
further addition of a very small percentage of mercury 
to increase the durability, has also been tried, but its 
use has proved detrimental; and the antimony, though 
advantageous, as a flux, is not so durable as the lead. 

The Julien Cell. — Various improvements of the Faure 
cell have been attempted, the chief objects of which have 
been to obtain greater durability, reduced weight, and 
to prevent the buckling of the plates. Prominent 
among these is the cell of Edward Julien of Belgium, 
whose general construction is similar to that of the im- 
proved Faure cell. Its chief claim is a special composi- 
tion of superior durability for the grids, which, so far 
as can be ascertained, is 94.5 per cent lead, 4.2 per cent 
antimony, and 1.3 per cent mercury. 

The Pumpelly Cell. — A cell has recently been invented 
by J. K. Pumpelly of Chicago, the principal features of 
which are a horizontal position of the plates, supporting 
material between them, copper electrodes centrally 
located on opposite sides of the cell and reaching to its 
bottom, and a containing vessel of light durable mate- 
rial. The construction, in other respects, is similar to 
that of the Faure cell, and the same materials are em- 



248 D YNA MIC ELECTRICl T Y A ND MA GNE TlSM. 

ployed for the grids, paste, and fluid. A slight burr at 
the narrow part of the grid openings holds the paste 
more securely, 12 per cent of antimony enters into the 
composition of the grids, and 20 per cent of sulphuric 
acid into that of the fluid. 

Fig. 89 shows the construction. The positive and 
negative plates alternate in position, the top and bottom 
plates being negative, and they are supported and in- 
sulated by cellulose made from 
wood, said to be a good insulator, 
an excellent absorbent, and in- 
destructible in sulphuric acid. 
Each plate has, at the centre of 
one of its edges, an opening about 
an inch square, and, at the same 
point on the opposite edge, a 
round, vertical, tubular projection 
about an inch high and half an 
inch in diameter, on the under 
^* side of which is a small socket 

fitted to the upper end of a similar projection frotn 
the alternate plate underneath. 

When the plates are built up in the cell, with the 
cellulose between them, each set has the projections all 
on the same side and the openings on the opposite side; 
the projections of each alternating with the openings 
of the other on the same side; so that each projection 
from a negative passes up through an opening in a posi- 
tive, with ample space for insulation, and helps to sup- 
port the next negative above; and each projection from 
a positive passes similarly to the next positive through 
an opening in the intervening negative. These projec- 
tions form a continuous tube on each side, from top to 
bottom, in which are placed the copper electrodes, and 
melted lead is poured in around them, giving perfect 




ELECTRIC STORAGE. 240 

metallic contact, and holding each set of plates firmly 
in position. 

The plates, thus built up, are immersed in the fluid in 
a hard-rubber vessel, rest on wooden blocks, and are 
charged, without reversal of current, in the cell designed 
for use. The E. M. F. is about 2 volts. 

Durability of Storage Cells. — Manufacturers usually 
guarantee for the positive plates a durability of one 
year in constant practical use, with a normal current. 
The negatives are far more durable, not being subject 
to oxidation; and, unless injured by buckling, last for 
an indefinitely long period. 

Storage Capacity. — The storage capacity of the 15-L. 
Faure cell, or the 300-ampere Pumpelly cell, is about 
1,080,000 coulombs, equal to 30 ampere-hours. Hence 
such a cell may be discharged in i hour with a 300-am- 
pere current, or in 10 hours with a normal, 30-ampere 
current; the time in seconds or hours, for a normal dis- 
charge, being estimated at ^ of the storage capacity in 
coulombs or ampere-hours. 

Relative Time of Charging and Discharging. — The time 
required for charging a cell is estimated at 18 to 20 per 
cent more than that required for discharging it with 
the same current strength; that being the usual per- 
centage of loss of energy between the charge and dis- 
charge. Hence if a 300-ampere cell is discharged in 
10 hours with a 30-ampere current, 12 hours would be 
required to charge it with a 30-ampere current, or 36 
hours with a lo-ampere current. The current strength 
required for charging is estimated at 5 amperes per 
square foot of positive plate surface. 

The preparatory charging of the Pumpelly cell at the 
factory occupies only the same time as each subse- 
quent charging in actual use. Hence only about 



250 DYNAMIC ELECTRICITY AND MAGNETISM. 

of the litharge on the negatives is reduced, at first, to 
spongy lead; the remainder being gradually reduced by 
use; which probably accounts in part for the fact ob- 
served, that the cell increases in energy during the first 
six months of use. 

The Hydrogen Alloy Theory. — A new theory of the 
electrochemical action of accumulators has been pro- 
posed by Dr. Paul Schoop, based on the following facts: 
It has long been known that certain metals, as plat- 
inum, palladium, mercury, and iron, combine, under 
certain conditions, with hydrogen; and on the theory 
that hydrogen is a metal, these combinations are regarded 
as alloys. 

It is also well known that when platinum sponge, 
charged with hydrogen, is exposed to the air it be- 
comes rapidly heated to redness by the absorption of 
oxygen ; also that the charged cathode plate of an 
accumulator, when similarly exposed, has its tempera- 
ture raised, from the same cause, often to the melting 
point. Hence Dr. Schoop assumes that the spongy lead 
of the cathode, like the platinum sponge, absorbs the 
hydrogen liberated from the solution by the electric 
current during the charging; the hydrogen combining 
with the lead and forming an alloy, and the liberated 
oxygen combining with the material in the anode and 
forming the lead dioxide: and that during the discharge, 
oxygen liberated by the current from the dioxide com- 
bines with the hydrogen of the cathode, reducing the 
alloy to spongy lead and restoring water to the solu- 
tion; leaving the material in the anode with the same 
proportion of oxygen as before charging. 

This theory is simple, but defective in failing to ac- 
count for the formation of the lead sulphate, and its 
varying proportions during the charging and discharg- 
ing. It is not easy to see how hydrogen can unite with 



ELECTRIC STORAGE. 25 I 

spongy lead incrusted with sulphate; so that unless the 
formation of sulphate, under normal conditions of the 
cell, be ignored, or a cell produced from which its for- 
mation shall be eliminated, the correctness of this 
theory must be considered questionable. 



2 $2 DYNAMIC ELECTRICITY AND MAGNETISM, 



CHAPTER X. 
THE RELATIONS OF ELECTRICITY TO HEAT. 

The mutual relations of heat and electricity are 
among the most important in electric science, whether 
considered with reference to the generation of elec- 
tricity, its transmission, its measurement, or its numer- 
ous forms of practical application. There can be no 
expenditure of electric energy without the simultaneous 
development of heat ; and it may also be assumed, 
though not so manifestly proved, that there can be no 
expenditure of heat energy without the simultaneous 
development of electricity. 

Heat Developed by Electric Transmission. — According 
to the best evidence we have, electricity and heat are 
different kinds of molecular motion, and the transmis- 
sion of either is simply the extension of this motion 
through a material substance connected with the gen- 
erator, knowm as the conductor. When electricity is 
thus transmitted, its transmission is always attended by 
the evolution of heat, which must be considered a legit- 
imate part of the work done, whether useful or other- 
wise, and not a mere adjunct. 

This heat is found to be always in direct proportion 
to the electric resistance encountered; hence if the use- 
ful work to be done is the production of heat, or its 
concomitant, light, the resistance is increased at the 
point w^here the heat or light is required: but if other 



THE RELATIONS OF ELECTRICITY TO HEAT. 253 

useful work is to be accomplished, the heat is suppressed 
by lessening the electric resistance, as required. Thus 
the ratio of heat work to other work can be made to 
vary by varying the resistance. 

The analogy to this is found in the friction attendant 
on mechanical action, which may produce heat for a 
useful purpose, or be suppressed by the use of a lubri- 
cant when the mechanical energy is to be otherwise 
expended. 

Joule's Law. — To determine accurately the relations 
between the electric current and the heat developed by 
it, Joule, who made this branch of electric science a 
specialty, passed a battery current through a tine wire 
coil inclosed in a vessel of alcohol, in which was also in- 
serted a thermometer. 

The resistance and current strength being known, 
were compared with the temperature to which the liquid 
was raised in a given time, and by this means were es- 
tablished the facts embodied in the following law: 

The heat developed in a conductor by an electric current 
passing through it varies as the conductor's resistance, 

the SQUARE OF THE CURRENT'S STRENGTH, and the TIME 
THE CURRENT LASTS. 

Representing the heat by ZT, the current by C, the 
resistance by i?, and the time by /, we get Zr= C^Rt as 
the algebraic expression of this law, by which the heat 
developed in any electric circuit can be ascertained. 

Joule's Equivalent. — Joule found that the amount of 
heat required to raise the temperature of i gramme of 
water 1° C. is equivalent, in work, to 42,000,000 ergs in 
C. G. S. measure; and this is known b,-;, Joule s equivalent. 
When the heat is produced by an electric current, the 
formula given above must be multiplied by 0.238 to re- 
duce the electric C. G. S. units to heat units; that being 
the ratio, expressed decimally, of 10,000, oco, the C. G. S. 



f 

254 D YNAMIC ELECTRICITY A ND MA GNE TISM. 

value of the electric units represented by C^Rt, to 42,000, 
000 (10,000,000 -f- 42,000,000 =: 0.238), and the formula is 
thenZr= C'Rt X 0.238. 

Heat Developed by Electrochemical Action. — The ex- 
periments of Favre on the electrochemical development 
of heat fully establish the correctness of the principle, 
that the evolution of heat by electric action is in the 
inverse ratio of other work accomplished by the same 
action; and that the heat developed in the battery cir- 
cuit is the exact equivalent of the chemical energy 
expended in the cells, as first verified approximately 
by Joule. 

In these experiments he used a mercurial calorime- 
ter, so constructed that the mercury should surround 
the apparatus in which the heat was to be generated, 
and by its expansion register the amount of heat de- 
veloped. Placing in this instrument a vessel containing 
zinc and sulphuric acid, he found that the simple 
chemical consumption of 2)2> grammes of zinc produced 
18,682 units of heat. He then replaced this vessel by a 
Smee battery cell, and noted the electrochemical con- 
sumption of the same amount of zinc ; varying the 
experiments by using connecting wires of different 
resistance, and also by comparing the evolution of heat 
when the cell was placed in the instrument and the 
connecting coil was outside, with its evolution when the 
coil was placed in the instrument and the cell was out- 
side. The results varied but slightly from that of the 
first experiment, the consumption of -i^^^ grammes of zinc 
producing 18,674 units of heat. The first experiment 
showed the amount of heat developed by a given amount 
of chemical action, measured by the consumption of the 
zinc; and the second proved that practically the same 
amount of heat was developed in the battery circuit by 
this amount of chemical action in the cell. 



THE RELATIONS OF ELECTRICITY TO HEAT. 255 

To show the mutual relations between electric heat 
and other electric work, a battery of 5 Smee cells, joined 
in series, was placed in the calorimeter, and connected 
with a small electromagnetic engine; and the evolution 
of heat during the consumption of ^iZ grammes of zinc 
noted in three different experiments, as follows: i. With 
the engine at rest the heat evolved was 18,667 units. 
2. With the engine running, but doing no work except 
to overcome its own friction and inertia, the heat 
evolved was 18,657 units. 3. When the engine by raising 
a weight did 12,874,000,000 ergs of work, the heat evolved 
was 18,374 units. Dividing the number which repre- 
sents the work in the last experiment by Joule's equiva- 
lent (42 X 10^) gives 306 heat units, and 18,374 + 306=1 
18,680. Hence, with proper allowance for unavoidable 
discrepancies, we find that in the last three experiments, 
as in the first two, the heat evolved was equivalent to 
the chemical energy expended; while the last experi- 
ment proved that the evolution of heat is in the inverse 
ratio of other work; the heat which disappeared being 
reproduced as work; a result conformable to the doc- 
trine of the conservation of energy. 

Electro-Thermal Capacity of Conductors. — Since the 
heat developed in a conductor by an electric current 
varies as the resistance, and the resistance varies with 
the nature of the material, and also directly as the length 
and inversely as the cross-section of the conductor, it 
follows that material^ mass^ and ratio between length and 
cross-section must each be considered in estimating the 
conductor's electro-thermal capacity. 

In conductors of equal length and cross-section but 
different conductivity, the heat developed in each by the 
transmission of currents of equal strength varies in- 
versely as the conductivity, or, which is the same, 
directly as the resistance due to difference of material. 



256 DYNAMIC ELECTRICITY AND MAGNETISM. 

Thus german-silver having about 13 times the electric 
resistance of copper, the heat developed in a german 
silver wire would be about 13 times that developed in a 
copper wire of the same dimensions, carrying a current 
of equal strength. 

But in conductors of the same material and mass, the 
resistance, and hence the heat development, varies 
directly as the ratio of length to cross-section, and in- 
versely as the ratio of cross-section to length. Suppose 
100 units of heat to be developed by the passage of a 
current through a wire 10 feet long, then only 10 units 
would be developed by the same current in a section of 
the same wire i foot long; hence if the wire be re- 
garded as made up of 10 sections arranged in series, 
10 units is the amount developed in each section suc- 
cessively. Now suppose a current of the same strength 
passed through a wire of the same material and mass, 
I foot long; the cross-section of this wire would evi- 
dently be 10 times as large as that of the other wire, 
consequently the resistance and heat development 
would be only ^, that is 10 units; the effect being the 
same as if the current passed through the 10 sections in 
parallel. But as the 10 units are equally distributed 
through the mass, only i unit of heat is developed in 
each section; that is, -^-^ the amount developed in each 
section, or same mass, of the long wire, or series con- 
nection. 

Suppose a current of the same strength passed through 
a wire of the same material and mass, 100 feet long; the 
length being 10 times that of the original wire, the cross- 
section would evidently be only -^-^\ hence the resistance 
and heat development would be 10 times as great, equal 
to 1000 units, or 10 units to each foot. But these 10 units 
being developed in -^-^ of the original mass per foot 
would raise the temperature to 10 times the original 



THE RELATIONS OF ELECTRICITY TO HEAT. 257 

temperature per foot, or 100 units. Now since the cross- 
sections of wires vary as the squares of their diameters, 
and the heat development varies inversely as the cross- 
section, ^ig- the cross-section producing 10 times the heat, 
it is evident that the rise of temperature in a conductor^ 
or heat developjnent per unit of inass, varies inversely as the 
fourth power of the conductor's diameter. 

The heat development per unit of mass^ as illustrated by 
the last example, deserves special notice. The number 
of heat units developed in a foot of the ten-foot wire 
was found to be just the same as in a foot of the hun- 
dred-foot wire, 10 units in each, though the rise of tem- 
perature in the last was 10 times as great, being in- 
versely proportional to the reduction of mass. Hence 
each wire, if immersed in an equal mass of the same 
liquid, to which its 10 heat units should be imparted, 
would produce the same rise of temperature, as indi- 
cated by a thermometer; for though the section of small 
wire becomes 10 times as hot as that of the large wire, 
it has only -^ of the mass, and hence only the same 
heating power. 

These principles have a highly important useful ap- 
plication, especially in electric lighting, which will be 
separately considered in a future chapter; but there are 
several minor applications, some of the more important 
of which may be noticed here. 

Electric Blasting. — The explosion of a blast can be 
safely and expeditiously effected at any required dis- 
tance, by inclosing a fine wire of high resistance, usually 
platinum, in the fuse, and connecting it with a battery 
circuit of low resistance, conveying a current of the re- 
quired strength. When the circuit is closed, the cur- 
rent, which produces scarcely a perceptible change of 
temperature in the main conductor, instantly raises the 
platinum wire to a white heat, producing the explosion. 



25^ DYNAMIC ELECTRICITY AND MAGNETISM. 

In this way blasts under water are fired, and mines and 
torpedoes exploded. The explosion of the great blast 
under the ledge of rock in Hellgate, New York harbor, 
by the touch of a child's finger closing the circuit, is a 
noted instance of this. 

Electric Cautery. — In surgery a fine platinum wire, 
heated to incandescence by an electric current, is pre- 
ferred to the knife for certain purposes; the operation, 
which is known as electric cautery^ being more safely and 
expeditiously performed in this way; as in amputation 
of the tongue for cancer, the removal of an excrescence, 
or of superfluous hair from a lady's face. 

Electric Fuses. — As conductors carrying strong cur- 
rents are liable, from accidental causes, to become over- 
heated and ignite inflammable matter in close prox- 
imity, a short section of a soft compound metal of high 
resistance, technically known as a fuse^ is introduced 
into the circuit at any convenient point. The cross- 
section of this fuse is so adjusted to the normal strength 
of the current carried, that if, from any abnormal in- 
crease, the temperature approaches an unsafe degree, 
the fuse melts and opens the circuit. 

The metals forming the compound are usually lead, 
tin, bismuth, and antimony, combined in different pro- 
portions according to the melting temperature, and 
other properties required. Fuses are usually from \ to 
f of an inch long, and from ^^ to ^^ of an inch in diame- 
ter, and adjusted to carry currents of from \ an ampere 
to 200 amperes, without fusion; the melting temperature 
being made, by adjustment of cross-section, about 20 
per cent above the carrying temperature in the large 
fuses, and about 50^ above in the small ones, when 
inclosed. The reason of this is found in the nature 
of the composition required for each; bismuth and 
lead, which melt at a comparatively low temperature, 



I'lJE RELATIONS OF ELECTRICITY TO HEAT 259 

entering largely into the composition of the small fuse 
to give it the requisite tenacity, while tin and antimony, 
which have a higher melting temperature, but are more 
brittle and less expensive than bismuth, predominate in 
the large fuse, in which there is less risk of fracture, 
and in which economy of material is less of an object. 
In the open air the melting temperature of the large 
fuses is about 5^ higher than when inclosed, and that of 
the small ones about 8^ higher. 

As the conductivity of metallic conductors decreases 
with rise of temperature, and as the radiation of heat 
increases with increase of cross-section, both these 
points must also be considered; so that the proper con- 
struction of fuses, including material, cross-section, 
carrying capacity, and melting temperature, adapted to 
varying conditions, is a difficult scientific problem, and 
one of great practical importance. If a fuse melts too 
easily it becomes a source of annoyance from frequent, 
unnecessary interruption of current, while if its tem- 
perature of fusion is too high it fails to afford protec- 
tion against fire. 

Fuses are connected by binding-screws to insulating 
blocks, to which the terminals of the conductor are also 
similarly attached, and hence are easily replaced at a 
nominal expense when melted; several fuses, connected 
with different circuits, or different branches of a circuit, 
being often arranged in the same block. 

Thermo-Electric Generation. — Before entering fully 
upon the consideration of thermo-electric generation, it 
is important to present certain general principles of 
electric generation which have a special bearing on this 
branch of our subject. 

An examination of the various kinds of apparatus 
by which electricity is generated shows that the con- 
struction, in every case, involves the following con- 



26o DYNAMIC ELECTRICITY AMD MAGNETISM. 

ditions: i. A complete insulated electric circuit composed oj 
heterogeneous materials. 2. Molecular excitation at one or 
more points in this circuit. And it may be safely assumed 
that in all cases where these conditions are fulfilled, 
either by natural or artificial means, electricity is gen- 
erated, even though such generation may not be appar- 
ent. 

These conditions are a legitimate consequence of the 
law of the conservation of energy as applied to elec- 
tricity considered as a mode of molecular motion. For 
if the circuit were not complete, molecular motion, ex- 
cited at any point, must soon cease; for the continuous 
storing of energy in one place implies its removal from 
another place, and this cannot continue indefinitely, nor 
for any considerable time, without a connection between 
the two places by which the transferred energy can re- 
turn to the place of its origin. The same would be true 
if the circuit w^ere complete but perfectly homogeneous 
throughout, both as to material and resistance, for mo- 
lecular motion would then be transmitted equally in 
opposite directions, and the meeting of these equal, 
opposing currents would stop the flow, producing a re- 
sult similar to that in the former case. 

But if, from difference in the nature of the materials, 
or in their resistance, or both, molecular motion is more 
free to extend itself in one of two opposite directions 
than in the other, and by a transfer of energy, incident 
to such extension, there occurs a corresponding reduc- 
tion of such motion in the opposite direction, that is, 
in electric language, if the current becomes positive in 
one direction and correspondingly negative in the oppo- 
site, it is evident that this motion must extend itself 
round the circuit continuously, or the current continue 
to flow from higher to lower potential, so long as the 
exciting cause continues; the transferred energy, which 



THE RELATIONS OE ELECTRICITY TO HEAT. 26 1 

produces the molecular motion, being again restored to 
the place of its origin. Just as water in a circular 
trough, receiving a continued impetus in the same 
direction at any point, would flow round continuously. 

This is precisely what occurs in a battery circuit or in 
the circuit of an electrostatic machine; materials differing 
in molecular constitution and resistance, as brass, glass, 
hard-rubber, pointed conductors and spherical conduc- 
tors, in the machine, and zinc, fluid, carbon or its equiv- 
alent, and copper, in the battery, forming the circuit, 
which is so arranged in each case that electric action, 
beginning at a certain point of junction of different 
materials, is continuously transmitted more easily in one 
direction than in the opposite; mechanical action being 
the exciting cause in the machine and chemical action 
in the battery; and the energy, whether mechanical or 
chemical, thus absorbed, reappearing as electricity. 

We may now consider the application of these prin- 
ciples to thermo-electric generation. In 1821 Seebeck 
made the discovery that an electric current could be 
generated by heating or cooling the junction of two dis- 
similar metals connected in an electric circuit. See- 
beck's experiment may be repeated by soldering or 
fusing together the ends of short pieces of any two 
metals, differing materially in molecular constitution, 
as bismuth and antimony, or german-silver and iron, 
and connecting their free ends, electrically, with a deli- 
cate galvanometer. On heating the junction to a tem- 
perature above the rest of the circuit, by a spirit-lamp 
or otherwise, the needle will be deflected, showing the 
generation of an electric current, and the same effect, 
with reversed current, will be produced if the junction 
be correspondingly cooled, as may be done by the appli- 
cation of ice; the direction of the current when the 
junction is heated being from bismuth to antimony, and, 



262 D YNA MIC ELEC TRtCI TY A ND MA GJVE TlSM. 

when cooled, from antimony to bismuth: the E. M. F., 
or potential difference, being in proportion to the differ- 
ence of temperature between the junction and the other 
parts of the circuit. Hence, in such a combination, 
composed of one or more couples, if each alternate 
junction be heated and the intervening junction simul- 
taneously cooled, the E. M. F. is proportionally in- 
creased, the current being from bismuth to antimony 
across each heated junction, and from antimony to 
bismuth across each cooled junction, and hence in the 
same direction round the circuit; and the same would 
be true of a circuit composed of any other metals hav- 
ing similar molecular differences. 

As the capacity of bismuth for heat is much lower 
than that of antimony, its rise of temperature with 
the same increment of heat is proportionally greater, 
and also its fall of temperature with the same abstrac- 
tion of heat; and as we find that the electric current 
flows from bismuth to antimony across the heated junc- 
tion, and oppositely across the cooled junction, it is 
evident that its flow in each case is from the hotter to 
the colder metal. But we know that the flow of an 
electric current is always from higher to lower potential, 
and in the direction of least resistance, and also that 
rise of temperature in a metal increases its electric re- 
sistance; hence we must infer that, in this case, increase 
of potential and resistance accompany rise of temper- 
ature, and decrease of potential and resistance accom- 
pany fall of temperature in each metal respectively, 
creating a preponderance of both in the hotter metal. 

On the molecular theory, it is the propagation of 
molecular motion, with heat as the exciting cause, which 
constitutes the electric current; heat undulations being, 
in some occult manner, transformed into electric un- 
dulations. Only a part of the heat supplied is thus 



THE RELATIONS OF ELECTRICITY TO HEAT. 263 

transformed, the remainder being either radiated, or 
appearing as heat in elevation of temperature in the 
circuit; and likewise when heat is abstracted, the re- 
maining heat, set in motion toward the junction by the 
cooling, is in part transformed into electricity, while the 
remainder is either radiated, or appears as heat in the 
reduced temperature of the circuit. 

In the above experiment the complete circuit is com- 
posed of three metals, copper forming the galvanometer 
coil and connections, though the generating part, or 
thermal battery, as it might be termed, is composed of 
only two metals. But the experiment may be performed 
with a circuit composed strictly of but two metals. For 
this purpose let a frame be constructed with a strip of 
copper or iron, of any convenient length, having its 
ends bent and soldered to a parallel bar of bismuth; or 
let it be bent so as to have parallel sides, and its free 
ends be connected by a cube of bismuth soldered to 
each. If this frame be mounted on an insulating stand, 
and a magnetic needle poised on a pivot at its center, 
the needle will be deflected by heating or cooling one of 
the junctions, or by heating one junction and cooling 
the other, as in the former experiment. 

No thermo-electric current can be generated in a cir- 
cuit composed of a single metal of perfectly homoge- 
neous molecular structure; but even with a slight dif- 
ference, such as may be produced by a coil or twist in a 
wire, a perceptible current may be obtained, which be- 
comes more marked with increased difference of struc- 
ture, as between differently manufactured kinds of the 
same metal, having different degrees of hardness, brit- 
tleness, or tenacity: and with continued increase of 
molecular difference, as between different metals, thermo- 
electric development increases in like proportion. Which 
proves that this development is dependent on molecular 



264 DYNAMIC ELECTRICITY AND MAGNETISM. 

Structure, indicating an intimate relation, if not actual 
identity, between electricity and molecular motion. 

It is found that lead shows no perceptible difference 
of thermo-electric potential at different temperatures, 
like other metals; hence it has been chosen as the stand- 
ard by which the relative thermo-electric potentials of 
other metals may be compared. In making such com- 
parison the same mean temperature must be chosen for 
the various metals, since the relative thermo-electric 
potentials of different metals varies greatly at different 
temperatures. Taking the microvolt dooooo o^ ^^ ^ volt) 
as the unit of potential, and 1° C. as the heat unit, the 
following metals, at a mean temperature of 19° to 20° C, 
show, according to Matthiesen, the relative thermo- 
electric potentials indicated, in microvolts, when the 
temperature of the junction between any two of them is 
raised 1° C. above the rest of the circuit: 

Bismuth, Commercial, Pressed Wire 97 

Bismuth, Pure, Pressed Wire „ 89 

Bismuth, Crystal , Axial 65 

Bismuth, Crystal, Equatorial 45 

Cobalt 22 

Mercury .418 

Lead o 

Tin — .1 

Copper, Commercial — .1 

Platinum — .9 

Gold — 1.2 

Antimony, Pressed Wire o. — 2.8 

Silver, Pure Hard — 3 

Zinc, Pure Pressed — 3.7 

Copper, Electrolytic — 3.8 

Antimony, Commercial, Pressed Wire — 6 

Arsenic — 13-5^ 

Iron, Soft — 17.5 

Antimony, Axial — 22.6 

Antimony, Equatorial — 26.4 

Tellurium — 502 

Selenium — 807 



THE RELATIONS OF ELECTRICITY TO HEAT. 265 

Thus any two or more of these metals, arranged in 
this order in a series, would acquire this relative poten- 
tial difference with a heat increment at the junction, or 
junctions, of 1° C, and a mean temperature of 19° to 
20° C.; each being electropositive to all that follow, 
and electronegative to all that precede it. The potential 
difference between bismuth and cobalt, for instance, is 
97 — 22 = 75, and between copper and antimony, 26.4 — 
3.8=22.6, while between bismuth and antimony it is 
97 -j- 26.4 = 123.4; difference between any two above or 
below zero being obtained by subtraction, while differ- 
ence between one above and one below is obtained by 
addition. 

In a series composed of any of these metals, arranged 
as above, the entire potential difference, or thermal 
E, M. F., is found, as in a battery series, to be equal to 
the sum of all the differences between each separate 
couple. Each couple may thus be regarded as a ther- 
mal cell, or element, the two metals corresponding to 
the two electrodes in a battery cell, heat energy, instead 
of chemical energy, being the exciting cause. Hence 
in any similar series of metals, ABCD, the sum of the 
potential differences between each couple, as A and B, 
B and C, C and D, is the same as the potential differ- 
ence, or E. M. F., between the extremes A and D\ so 
that if a direct junction were made between A and Z>, 
and the intervening couples omitted, the E. M. F. would 
be the same as in the full series, as may be verified 
numerically in any series chosen from the table. 

In a circuit composed of several couples of any two 
metals, alternate junctions require to be either heated 
or cooled, or each alternate junction heated and the in- 
tervening one simultaneously cooled, as the heating or 
cooling of two adjacent junctions to the same tempera- 
ture would produce opposite, neutralizing currents. 



266 DYNAMIC ELECTRICITY AND MAGNETISM. 

The table is not intended to embrace all the sub- 
stances which manifest electro-thermal properties, but 
only a few of the metals in which those properties are 
prominent; such properties being common to a large 
class of bodies, both metallic and non-metallic. The 
potential differences given must be understood to apply 
only in a general sense, as differences of molecular 
structure produce, as shown, great variation in this re- 
spect; so that in different experiments the results ob- 
tained from the same metals procured from different 
sources would be only approximately the same. 

Thermo-Electric Diagrams. — Sir William Thomson pro- 
posed a graphic representation of the relative thermo- 
electric potentials of different substances at different 
degrees of temperature, consisting of a diagram having 
vertical lines representing the differences of tempera- 
ture, and lines approximately horizontal representing 
the thermo-electric differences of potential, and in 1856 
used such a diagram for the first time to illustrate a 
lecture. Fig. 90 shows a diagram by Prof. Tait con- 
structed in this way. Lead, being thermo-electrically 
constant at different temperatures, is represented by a 
perfectly horizontal line, marked o, while the other 
metals are represented by lines tilted at the various 
angles required to show their relative thermo-electric 
differences, at different temperatures, with respect to 
lead, and hence with respect to each other. Lines rep- 
resenting metals whose potential difference with respect 
to lead increases negatively or decreases positively with 
increase of temperature descend from left to right; ■ 
while those representing metals whose potential differ- 
ence increases positively or decreases negatively with 
increase of temperature ascend from left to right. Thus 
zinc, at 19° to 20° C, is shown to be about —3.7 below 



THE RELATIONS OF ELECTRICITY TO HEAT. 26/ 




268 DYNAMIC ELECTRICITY AND MAGNETISM. 

lead, as given in the table, while at 480° C. it is —15' 
below. 

Thermo-electric differences may be represented also 
by the areas formed by the lines. Thus in a zinc-iron 
couple, with one junction at 100° C. and the other at 
0° C, the thermo-electric difference is represented in the 
diagram by the area d^ b, c, d. For small tempera- 
ture differences, of one or two degrees, the superficial 
contents of the areas are practically the same as be- 
tween rectangles, and hence are obtained by simply 
multiplying the temperature differences by the poten- 
tial differences; but for large temperature differences 
the irregular shape of the areas requires special calcula- 
tion in each case. 

The Peltier EiFect. — Peltier's observations on the ther- 
mo-electric circuit led him to the natural conclusion 
that if difference of temperature at the junctions could 
generate an electric current, then, conversely, the pas- 
sage of an electric current through such a circuit must 
generate a corresponding difference of temperature at 
the junctions, and experiments made by him in 1834 
verified this conclusion. Hence this phenomenon, which 
is now a well-established thermo-electric law, has been 
called the Peltier effect^ in distinction from the genera- 
tion of heat by the resistance of the circuit, as observed 
by Joule, which is known as the Joule effect. 

These two effects are entirely consistent with each 
other and may occur simultaneously in the same cir- 
cuit. For, in any circuit, whether composed of one 
metal or several, the temperature varies in proportion 
to the square of the current's strength, in accordance 
with the Joule effect; but if the circuit is composed 
of different metals, or different kinds of the same 
metal, there occurs also a transfer of heat from one 
junction to another in proportion simply to the cur- 



THE RELATIONS OF ELECTRICITY TO HEAT. 269 

rent's strength, so that one junction is heated while 
the other is correspondingly cooled, in accordance with 
the Peltier effect. In the Joule effect the amount of 
heat generated in the circuit, as a whole, is not varied 
by the direction of the current, while in the Peltier 
effect the transfer of heat is reversed by reversal of 
current; so that junctions heated by a current flowing 
in a given direction, as from antimony to bismuth, are 
correspondingly cooled by the same current flowing in 
the opposite direction, as from bismuth to antimony, 
while the alternate junctions cooled in the first instance 
are correspondingly heated in the second. The reduc- 
tion of temperature in a bismuth-antimony combination 
may thus become so great as to freeze water in a cavity 
at the cooled junction. 

Thermo-Electric Inversion. — Prof. J. Gumming found 
that, in a copper-iron couple, iron ceases to be electro- 
negative to copper when the temperature of the junc- 
tion is raised to 280° C; the current from copper to 
iron ceasing, and the Peltier effect also disappearing, 
when a current is transmitted in either direction. But 
when the temperature of the junction is raised above 
280° C, iron becomes electropositive to copper and the 
Peltier effect is also renewed. This is illustrated by the 
iron and copper lines in the diagram which cross each 
other at the neutral point, iron being represented as 
electronegative to copper on the left of this point and 
electropositive on the right: similar thermo-electric 
inversion being also shown in other metals. 

The potential differences given in Tait's diagram vary 
somewhat from those given in Matthiesen's table, with 
which Cumming's experiments seem to accord more 
closely; only approximate accuracy being attainable in 
different experiments, for the reason already given. 

The Thomson Effect. — This thermo-electric inversion 



^70 DYNAMIC ELECTRICITY AND MAGNETISM. 

led Sir William Thomson to conclude that since there 
is no heat development, or Peltier effect, at the junction 
of a copper-iron couple, at 280° C, by the passage of an 
electric current through it, therefore, conversely, there 
can be no accumulation of heat at this point, at like 
temperature, when the current is generated by heat 
supplied to it, and therefore the heat supplied must be 
absorbed by other parts of the circuit than the junctions, 
and hence must pass between differently heated parts 
of the same metal. 

Experiments with different metals verified this con- 
clusion, showing that when a thermo-electric current 
passes through a conductor, from a hotter to a colder 
part, there is a transfer of heat, which in some metals, 
as copper, is from the hotter to the colder part, while 
in others, as iron, it is from the colder to the hotter 
part: but when the direction of the current is from a 
colder to a hotter part this transfer is reversed. This 
electric convection of heat in the same metal is known 
as the Thomson effect^ in distinction from the Peltier and 
Joule effects. 

It follows from the above that in a copper-iron cir- 
cuit, when a current is generated by heating a junction 
to any temperature below 280° C, the current being 
from copper to iron is from cold to hot in the copper 
and from hot to cold in the iron, so that in both metals 
heat is transferred to the junction; but when the tem- 
perature of the junction is raised above 280° C, the 
current being reversed, the heat transfer is also reversed, 
and is from the junction in both metals: while with the 
junction at 280° C, there being no current, there is no 
transfer of heat in either direction. And the same 
principles apply to any circuit having similar theraid- 
electric inversion. 

The Thermopile. — It has not yet been found possible 



THE RELATIOXS OF ELECTRICITY TO HEAT. 2/1 

to constiuct electric generators of general practical 
efficiency on the principle of the direct conversion of 
heat into electricity. Generators of this kind, con- 
structed by Clamond and others, have not fulfilled the 
hopes raised by their first apparent success; the genera- 
tion of strong currents, combined with the heat neces- 
sary to produce them, seems to effect, in a short time, 
such permanent change of molecular structure as to 
reduce the production and maintenance of potential 
difference between the different metals below the point 
of practical efficiency. 

The difficulties in such construction become further 
manifest when we consider that comparatively few of 
the metals given in the table are practically available 
for this purpose, either in consequence of small poten- 
tial difference, extreme rarity, as in the case of tellurium 
and selenium, volatility when heated, or other cause. 
Of the available metals, bismuth and antimony have the 
highest potential difference, and can be used at moder- 
ate temperatures; bismuth melting at 264° C. and 
antimony at 450" C. 

The ther?fiopile, represented by Fig. 91, is constructed 
with a number of small, short metal bars, usually of 
bismuth and antimony, arranged side by side in couples, 
junctions being formed between each pair of ends in 
alternate order, by soldering or fusing; the arrangement 
being such that the current must pass from one metal 
to the other through the entire series. By having as 
many layers as there are bars in a layer, a compact, 
cubical form is obtained. 

The bars thus arranged and properly insulated are 
inclosed in a brass case, open at the ends, and mounted 
on a stand provided w4th apparatus for elevating and 
adjusting them to any required height or angle. Coni- 
cal caps are fitted to the ends to admit the heat radiated 



2/2 DYNAMIC ELECTRICITY AND MAGNETISM. 

from any object whose temperature is to be tested, and 
to exclude radiation from other sources; and binding- 
screws are provided for the galvanometer connections. 
The alternate junctions being at opposite ends, one 
set may be exposed to heat while the alternate set are 
cooled; and the entire potential difference, or E. M. F., 
being equal to the sum of the potential differences in 




Fig. 91. 

the series, a very sensitive apparatus, for investigating 
slight differences of temperature, may be obtained, when 
the instrument is used in connection with a sensitive 
galvanometer; the needle responding instantly with a 
prominent movement, easily read from the scale, to 
temperature differences hardly perceptible in the ther- 
mometer: the heat generated by the bending of a 
copper wire being sufficient to produce a deflection of 
several degrees. The highly important investigations 
of Melloni and of Tyndal on heat were conducted with 
the aid of such an apparatus. 



THE RELATIONS OE ELECTRICITY TO HEAT. 2/3 

The relative E. M, F. of the thermopile as compared 
with other generators is very small. Taking 200 micro- 
volts as the average E. M. F. attainable by the simuL 
taneous heating and cooling of the opposite junctions 
in a single bismuth-antimony couple, the total E. M. F. 
in a thermopile, or multiplier, of 50 such couples would 
be 50 X 200 = 10,000 microvolts, or xoffoo"o — Too ^^ ^ 
volt; so that the combined E. M. F, of 100 such genera- 
tors would be only equal to that of a single Daniell cell. 
But as the comparative resistance of the thermopile is 
also small, the current is comparatively large: suppos- 
ing the resistance in the above case to be -jV of ai^ 

ohm, then ^"" = ^C, or -^ of an ampere. In the 

To"^ 

largest Clamond thermo-electric batteries, consisting of 
150 iron-galena elements, the estimated E. M. F. is only 
5i*o volts, and the internal resistance 2 ohms, which 
would give a current of 2-f^ amperes, in an external cir- 
cuit of no resistance. A Daniell battery of the same 
number of elements similarly joined, in series, would 
have an E. M. F. of about 150 volts and an internal 
resistance of about 300 ohms, which would give a 
current of \ an ampere, in a similar external circuit, 
less than \ that of the thermo-electric battery, though 
its E. M. F. is 28 times as great. 

Electric Welding. — This highly important application 
of electricity has been largely developed by Prof. Elihu 
Thomson since 1886, and has now attained a wide range 
of practical work. It consists in uniting pieces of metal 
by pressing them together, end to end, and heating the 
juncture by an electric current till the metal becomes 
sufficiently plastic to form a perfect joint; only so much 
of It being included in the circuit as may be necessary 
for this purpose. 

The alternating current is employed, and applied by 



274 DYNAMIC E'LECTkiClTY AND MAGNETISM, 



the welder shown in Fig. 92, which consists of a con 
verter and clamping apparatus combined. The con 
verter, shown in the rear, is constructed with a laminat- 
ed iron core inclosing a massive copper tube, equivalent 




Fig. 92. 

to a single coil, which forms the secondary circuit. The 
primary circuit consists of an insulated copper coil 
wound in two sections through the interior of this tube, 
as shown, and inclosing its upper and lower parts 
together with the adjacent parts of the core. This 
circuit is connected with the dynamo by the terminal 



THE RELATIONS OF ELECTRICITY TO HEAT. 2/'S 

wires shown in the rear, and the secondary circuit 
is connected, on the right, w4th a massive grooved 
copper bar, to which is fitted the copper sliding-block A. 
Two massive copper clamps, C and C, grasp the two 
bars to be welded together, the right one movable in 
connection with the sliding-block A, and the left fixed; 
and to this fixed clamp the secondary circuit is con- 
nected on the left. 

Pressure being applied to the block A by the crank jB 
and connected gearing, the right bar to be welded is 
forced against the left; the circuit being opened and 
closed by a switch connected with a treadle, and the 
current regulated by a reactive coil connected wdth the 
primary circuit. 

A dynamo, specially adapted to this w^ork, furnishes 
a current which, in the 20,000-watt welder, has an 
E. M. F. of 300 volts at the terminals of the primary 
circuit, which is reduced, in the secondary circuit, 
to about I volt ; and the efficiency being about 80 
per cent, the maximum current is about 16,000 am- 
peres. The welding capacity of a welder of this size, 
for bar-iron, ranges from bars f of an inch in diameter 
to bars of i^ inch diameter; the range for brass being 
three fourths of this, and for copper one half. 

To adapt the welder to different kinds of work, its 
primary circuit is connected in series to an auxiliary 
converter of special construction, by which the E. M. F. 
can be more fully controlled. The primary circuit of 
this converter is wound on a section of a laminated iron 
core, composed of a split ring, the slit being on the 
opposite side from the coils, which are so arranged that 
they can be joined either in series or parallel by a 
switch. The core incloses an iron armature, upon 
which is mounted the secondary circuit, consisting of a 
massive brass casting, which also includes a section of 



276 DYNAMIC ELECTRICITY AND MAGNETISM. 

the core and may be rotated so as to include either the 
primary circuit or the slit, as required. When rotated 
so as to include the primary, the E. M. F. is reduced to 
one half that which it is when the secondary is opposite 
the slit, across w^hich no lines of force can pass, and 
where the magnetism is therefore at its minimum. The 
E. M. F. can also be either increased or diminished by 
joining the coils of the primary, either in series or in 
parallel; hence its variation in the welder, by these vari- 
ous means, includes a wide range. 

The above, known as the indirect method, is em- 
ployed for the heavier and more complicated kinds of 
welding, and where several welders are operated by 
current from a single dynamo ; while for the lighter, 
simpler kinds the direct method is employed, in which 
the welder is connected directly to the dynamo, the 
armature of which has a high potential circuit of fine 
wire, in series with the field-magnet coils, which acts 
inductively on a low potential circuit, composed of a 
massive, U-shaped, copper bar, connected directly to 
the welding apparatus, the construction of which is the 
same as already described. 

The direct current may be employed, but the alter- 
nating is preferred on account of its higher efficiency 
and freedom from electrolyzing effects, a point of spe- 
cial importance in the welding of alloys. 

The ends to be welded together are rounded so that 
contact shall be first made at the center, and the weld 
being from the center outwards, oxidized particles and 
other impurities are forced out as the ends are pressed 
together, making a perfect joint, superior to any which 
can be made by forging; and the entire process being 
thus open to the inspection of the operator, flaws cannot 
escape observation. Manual pressure can be employed 
in ordinary cases, but for more difficult welding, where 



THE RELATIONS OF ELECTRICITY TO HEAT. 277 

great accuracy is required, pressure by hydraulic or 
other mechanical power is preferred. 

The greatest heat is developed at the center of the 
weld, extending only a short distance on either side, 
and varying directly as the resistance, which increases 
wnth the rise of temperature. Bars of inch iron become 
red hot for a distance of \\ inches on either side of the 
weld, but are comparatively cool at a distance of 2J- 
inches ; and the operation being completed in 40 
seconds, the time is too short for diffusion of the heat 
by conduction ; hence waste of energy from this source 
is reduced to the minimum. The time varies for metals 
of different kinds and sizes, from i or 2 seconds for fine 
wires to 2 or 3 minutes for heavy bars ; wrought-iron 
bars, 2 inches in diameter, requiring an average time of 
about 97 seconds ; 2j-inch iron pipes, \ inch thick, 61 
seconds ; the average time for copper bars being about 
"I that required for wrought-iron bars. 

The E. M. F. is so low that the enormous current re- 
quired for heavy work is perfectly safe; and conductors 
carrying currents of many thousand amperes, but having 
an E. M. F. of only a fraction of a volt, may be handled 
with impunity and without sensible effect. 

The range of application is almost unlimited, em- 
bracing not only all welding hitherto considered 
practicable, but a large amount considered either 
wholly impracticable or extremely difficult, ranging 
from the most refractory metals to alloys fusible at 
90° C. Not only can such metals as cast-iron, copper, 
lead, tin, zinc, brass, german-silver, and bronze be 
welded, each to its own kind, but any of these dissimilar 
kinds can be welded together. Steel cables composed 
of a large number of fine wires, tubing, and various 
kinds of metal work usually united by screws, rivets, 
soldering, or brazing, can be welded by this processj 



^7^ DYNAMIC ELECTRICITY AND MAGNETISM. 

also articles which, from their peculiar shape, are diffi- 
cult or impossible to weld in the ordinary way. It has 
also a highly important application in the expeditious 
repairing of broken machinery on ships, in factories, 
and elsewhere. 

Welds made by this process have been subjected to 
the severest practical tests by the United States naval 
authorities and various, eminent, civil and electrical 
engineers, and have received their unqualified approval 
for superior strength and tenacity. 



THE KELATWiVS OF ELECTRICITY TO LIGHT, 2/9 



CHAPTER XI. 
THE RELATIONS OF ELECTRICITY TO LIGHT. 

The Relations of Electric Heat to Electric Light. — It 

has been shown that heat is always a result of electric 
resistance, and is in proportion to such resistance; and 
as a certain degree of resistance is found in every con- 
ductor, it follows that heat always accompanies electric 
transmission. When the heat increases to a sufficient 
degree of intensity, light is produced, either by incan- 
descence or combustion according to the nature of 
the medium of transmission. Hence the electric genera- 
tion of light follows that of heat and is dependent on 
heat intensity; heat being produced without light, but 
light never being produced without heat. 

Heat and light, according to well-established theories, 
being considered different modes of molecular motion, 
if electricity also be so considered, the difference 
between the three would seem to consist in the nature 
of the motion in each case, and may be attributed to 
differences in the length, amplitude, rapidity or phase 
of undulation peculiar to each, as pertaining both to 
the molecules of the conductor, and to the medium of 
transmission through space. 

Neither phenomenon is developed at the expense of 
the other, except as the nature of certain conductors 
produces variation between the development of heat 
and electric current; hence if these are different kinds of 
molecular motion, they must occur in such a manner as 
not to neutralize each other. It has been shown how such 
different kinds of motion may coexist without interfer- 



2 So DYNAMIC ELECTRICITY AND MAGNETISM 

ence in the magnet, and similarly here, motion whose 
general direction is in lines, straight, curved, or spiral, 
would not interfere with transverse undulations, nor 
would either interfere with rotary motion of the mole- 
cules. 

It is not impossible that two or more of these phe- 
nomena may be identical; that the heat undulations, or 
the electric undulations, or both, are, at a certain degree 
of intensity, recognized as light; though, in the present 
state of our knowledge, it is more in accordance with 
observed facts to assign to each a distinct mode of 
motion; that of heat being comparatively slow, with 
considerable length and amplitude of undulation, while 
those of light and electricity are inconceivably rapid, 
with undulations of a corresponding character. 

We have seen that heat and electricity reproduce 
each other directly, but that in the electric production 
of light, heat intervenes, and that the light is appar- 
ently a result of the heat rather than of the electricity; 
for when the heat is produced by any other method, 
light usually follows increase of heat intensity in the 
same manner, though we have no direct evidence of 
the presence of electric action: and yet it is not im- 
possible that electricity, though occult, may be present 
as an active agent, or that the light and the electricity 
may be identical. 

Photo-Electric Generation. — While the direct generation 
of light by electricity is not clearly apparent, the direct 
generation of electricity by light has been effected ex- 
perimentally, though it has not yet been found possible 
to construct practical generators on this principle. 

The first experiment of this kind was made by Bec- 
querel about 1850, who found that when one of two 
silver plates, freshly coated with silver chloride and im- 
mersed in water, is exposed to light, an electric cur- 



THE RELATIONS OF ELECTRICITY TO LIGHT. 28 1 

rent, indicated by a connected galvanometer, flowed to 
the exposed plate from the opposite pole. 

In 1875-6 Adams and Day, English electricians, made 
a very extensive series of experiments to ascertain the 
electric relations of selenium to light; one result of 
which was the discovery of electric generation by this 
metal under the influence of light. A small piece of 
selenium, whose electric resistance had been reduced by 
annealing, had platinum terminals fused into its oppo- 
site ends; the platinum wire being formed into little 
rings on the inserted ends, to giver fuller contact. On 
exposure of the selenium to candle-light the passage of 
an electric current was indicated by a prominent deflec- 
tion in a connected galvanometer; the direction of the 
current being from the part least exposed to the part 
most exposed, a result similar to that in Becquerel's 
experiment. 

That this was not a thermo-electric current was 
proved in various ways: i. The current began promptly 
with the exposure, and ceased promptly with the ex- 
clusion of the light, instead of showing the more grad- 
ual increase and decrease of current due to heating and 
cooling. 2. The current, in most of the experiments, 
was the result of exposure of the body of the metal, 
while the thermo-electric current results from exposure 
of the junctions. 3. When the light was focused on a 
junction the direction of the current was from selenium 
to platinum, while that of a thermo-electric current 
would have been from platinum to selenium; this direc- 
tion being also, as will be perceived, from the least to 
the most exposed part of the selenium, as before. 

In 1887, Prof. Edlund constructed a generator by 
melting a very thin layer of selenium on a disk made of 
a metal with which it could unite chemically, and 
covering this layer with gold-leaf made so thin that the 



282 DYNAMIC ELECTRICITY AND MAGNETISM. 

sunlight could penetrate to the selenium. Connection 
with a galvanometer being made between the gold-leaf 
and lower disk, an electric current was developed on 
exposure to the sun's rays, which responded promptly 
to the influence of the lighl, and ceased promptly with 
its exclusion, thus proving its photo-electric character, 
as in the former example. 

Photo-Electric Reduction of Resistance in Selenium. — The 
electric resistance of ordinary, vitreous selenium is 
3.8 X 10^** = 38,000,000,000 times that of copper, but 
when annealed by being kept for several hours just be- 
low the point of fusion, 220° C, and then cooled slowly, 
it becomes crystalline and its resistance is materially re- 
duced. The difference of crystalline structure produced 
by the more rapid cooling of the exterior than the in- 
terior has been assigned by Gordon as a probable reason 
for its property of photo-electric generation. 

It was found by Adams and Day that the resistance of 
this annealed selenium, when a battery current is passed 
through it, is much less in the light than in the dark; 
the resistance varying directly as the square root of the quan- 
tity representing the illumination. 

Bell and Tainter utilized this property of selenium in 
the construction of \.\\€\r phot op hone. A narrow strip of 
selenium connected at the edges with broad plates of 
brass furnishes a photo-receiver of large surface ex- 
posure and of low resistance as respects form; the se- 
lenium furnishing a resistance highly sensitive to light 
and varying under its influence from 300 ohms to 150. 
This photo-receiver being placed in a battery circuit 
connected with a telephone receiver, the varying light 
reflected from a distant point by a thin mirror, con- 
stituting the disk of a telephone transmitter, which re- 
sponds to the undulations of the voice, produces corre- 
sponding variations in the battery current, by which the 



THE RE LA TIONS OF ELECTRICITY TO LIGHT. 283 

voice is reproduced in the telephone receiver, as ex- 
plained in connection with the telephone. 

Tellurium has the same photo-electric properties as 
selenium in less degree, and carbon also shows similar 
properties. 

Polorization of Light. — The ether undulations, supposed 
to constitute light, are believed to be transverse to the 
direction of the rays. This transverse undulation is 
supposed to be equal in all directions within a circular 
space, so that the theoretical conception of a ray viewed 
endways in cross-section would be that of a circle com- 
posed of an infinite number of planes of undulation in 
which the undulations, by mutual adaptation, occur 
without interference. As if numerous fine wires, each 
bent into short curves, in the same plane, at right angles 
to the wire's length, were fitted together so as to form 
a long slender cylinder, with these curves crossing each 
other at all possible angles along its central axis. 

But under certain conditions of transmission and re- 
flection, the ray becomes flattened, as if compressed be- 
tween opposite lateral forces, so that these undulations 
all occur in one plane, and the ray is then said to be 
polarized. 

This happens when light is transmitted through cer- 
tain crystals, especially tourmaline. If two thin plates 
of tourmaline be placed with their surfaces parallel to 
each other, and a ray of light be transmitted through 
them at right angles to their surfaces, and to a certain 
direction in each, known as its optical axis, the light 
will pass freely through both to a screen beyond, so long 
as these axes are parallel. But if either crystal be 
turned so that the optical axes are at an angle, the sur- 
faces being still parallel, the light which passes through 
one is obstructed in the other, gradually disappearing 
from the screen as the angle increases, till at 90° it is en- 



284 DYNAMIC ELECTRICITY AND MAGNETISM, 

tirely extinguished. If the rotation be continued in the 
same direction, the light gradually reappears on the 
screen, and regains its original brightness when the axes 
again become parallel. The crystal on which the light 
is first received is known as i\\^ polarizer 2,Xidi the other 
as the analyzer. 

The theory of this phenomenon is that the undula- 
tions in passing through the polarizer are changed from 
the phase of a circle to that of a plane, in which form 
they readily pass through the analyzer so long as the 
optical axes of both crystals lie in the same plane; but 
v/hen the planes of the axes cross, it is as impossible for 
the polarized light to pass through the analyzer as it 
would be for a metal rod, compressed into a sheet be- 
tween rollers, to pass crossways through the wires of a 
bird-cage. 

Light when reflected at certain angles from certain 
substances becomes polarized as well as when trans- 
mitted, the polarizing angle varying according to the 
nature of the reflecting substance. The analyzer in this 
case may be either a reflector or a transmitter, and the 
polarized ray is reflected, transmitted, or extinguished 
according to the angle at which it meets the analyzer. 

Magneto-optic Polarization. — Faraday's Discoveries. — In 
a series of experiments, made in 1845, Faraday found 
that polarized light is influenced by the electro- 
magnetic current. A polished piece of '' heavy glass" — 
silicated borate of lead — about 2 inches square and -|- 
an inch thick, was interposed edgeways in the path of 
a ray of lamp-light, polarized by reflection from a plane 
glass surface; the analyzer being turned so as to ex- 
tinguish the ray. A U electromagnet was placed close 
to the glass, in such position that a line through its 
poles, which were about 2 inches apart, was parallel to 
the direction of the ray. On the passage through its 



THE RELATIONS OF ELECTRICITY TO LIGHT. 2S5 

coils of an electric current from a battery of five Grove 
cells, the extinguished ray again passed through the 
analyzer, proving that its plane of polarization had been 
rotated into a new position by the electromagnetic ac- 
tion; which was confirmed by the fact that, by a further 
rotation of the analyzer, an angle was found in which 
the magnetized ray was extinguished, but in which the 
ray was transmitted when no current was passing — a 
reversal of the conditions of transmission and extinc- 
tion found in the first position. 

Faraday found that, to produce these results, a solid 
or a liquid medium of transmission was necessary for 
the reception of the magnetic action, but failed to ob- 
tain them by such action on air or other gaseous 
medium, or in vacuo. He also found that the direction 
in which the plane of polarization was thus rotated 
coincides with that in which the magnetizing current 
passes round the magnet ; reversal of current conse- 
quently producing reversal of this rotation. But it was 
subsequently ascertained by Verdet that this coincidence 
of direction is true only of diagmagnetic bodies, while, 
in certain paramagnetic bodies, this rotation is opposite 
to the direction of the magnetizing current. 

It should be especially noticed that the direction of 
the magnetic lines of force, from pole to pole, was, by 
the position given to the magnet, made parallel to the 
ray. 

Faraday varied his experiments by using different 
kinds and different forms of magnets, and placing the 
glass, or rather medium, in different relative positions; 
but to obtain the effect described, the parallel position 
of the ray to a line through the poles was requisite. 
He also, used a pair of bar electromagnets with tubular 
cores, so placed that a ray could be transmitted through 
both and received on any medium placed between dis- 



286 DYNAMIC ELECTRICITY AND MAGNETISM. 

similar poles, which, as in the U magnet, were about 2 
inches apart. 

Passing the ray horizontally across a single pole, with 
the magnet in a horizontal position, he found the ray's 
rotation, when the glass was on the side next the 
analyzer, to be the reverse of what it was when the glass 
was on the opposite side; change of pole or reversal of 
current producing reversal of rotation. But, with the 
glass above, below, or in front of the pole, no rotary 
effect was produced. The cause of these various effects 
becomes obvious when we consider that the lines of 
magnetic force radiate in all directions from a single 
pole: hence, when the glass was in the horizontal plane 
of the magnet, these curved lines, in that plane, were 
nearly parallel to the short portion of the ray trans- 
mitted through the glass, but radiated in opposite 
directions on opposite sides of the pole; so that on one 
side they coincided with the direction of the ray's trans- 
mission, and on the opposite side were opposed to it; 
but above or below the pole they were at right angles 
to the ray, while in front of it radiation was equal in 
opposite directions. 

Another rule given by Faraday for finding the direc- 
tion of the ray's rotation, with diamagnetic bodies, 
which has a special application to the case of a single 
pole, is substantially as follows: A ray of light, coming 
to the observer, is rotated in the same direction as 
watch-hands move, when the magnetic lines of force 
parallel to it are radiated from a north pole in the same 
direction as the ray, or from a south pole in the opposite 
direction; reversal of the ray's direction producing re- 
versal of rotation. 

Faraday obtained the same effect, in a limited degree, 
from steel magnets as from electromagnets; also from 
coils without iron cores; proving that the effect is chiefly 



THE RELATIONS OF ELECTRICITY TO LIGHT. 287 

magnetic, though also electric. He also found that this 
effect is independent of any specific polarizing property 
normally pertaining to the diamagnetic body through 
which the ray is passed; the electromagnetic polarizing 
effect being either increased or diminished by such 
specific property, according as it produced rotation in 
the same or in the opposite direction. He could not 
produce any change in this effect by any degree of mo- 
tion given to the dielectric while under the joint in- 
fluence of magnetism and light. He noticed that the 
rotation increased slowly, requiring about two seconds 
after the closing of the circuit for the attainment of the 
full effect, but that it ceased promptly on opening the 
circuit. The first result he attributes to a lag in the 
magnetic saturation of the core, while the second showed 
the intimate relation of this effect to electromagnetic 
action. His conclusion in regard to magnetic lag w^as 
confirmed by the fact that there was no lag when the coil 
alone, without a core, was used; the rotation respond- 
ing promptly both to the opening and closing of the 
circuit. He also found that any addition made to the 
dielectric on either side, and not in the line of the ray, 
produced no difference in the rotary effect. 

His final conclusion is, that since this effect is essen- 
tially the same in character under all these varying 
conditions, and is independent, in this respect, of the 
nature of the dielectric, or its own specific rotative force, 
therefore the magnetic force and the light have a direct, 
mutual relation, but require the intervention of matter 
as the medium of action. 

Verdet's Discoveries. — Experiments made by Verdet in 
1852 confirmed the results obtained by Faraday, except 
in regard to the direction of the rotation produced by 
certain paramagnetic bodies, as already explained. His 
apparatus consisted of two powerful electromagnets 



288 DYNAMIC ELECTRICITY AND MAGNETISM. 

with hollow cores, similar to those used also by Fara- 
day, through which light could be transmitted to the 
medium interposed between dissimilar poles, parallel to 
the lines of force. He also used a U electromagnet 
with massive, slotted pole-pieces, through which the 
light could be transmitted at any desired angle; the 
magnet having also a rotary movement by which the 
angle between the lines of force, from pole to pole, and 
the ray could be adjusted and measured with a gradu 
ated scale and vernier. The principal substances used 
as media were the " heavy glass," used by Faraday, 
common flint glass, and carbon bisulphide. 

Verdet endeavored to ascertain not only the facts in 
regard to electromagnetic polarization, but also the 
laws which govern it; and to determine the specific 
electromagnetic rotative force of different substances. 
His principal deductions are embodied in the following 
law: The rotation of the specific electromagnetic plane of 
polarization for any substance is directly proportional to the 
strength of the magnetic action, to the thickness of the ?nedium 
traversed joifttly by the magnetism and light, and to the cosine 
of the angle between the ray and the lines of magnetic force. 

Verdet chose water as his standard of comparison for 
specific rotative differences; but Gordon, who subse- 
quently made a special investigation of this subject, 
found carbon bisulphide a more reliable standard. 
Hence taking the specific magneto-rotative force of 
this substance ac unity, that of water is found to be 
0.308 and that of "heavy glass" 1.422. 

Becquerel's Discoveries. — These are the specific differ- 
ences for white light; but this force has been found to 
vary for different colored rays, and since difference of 
color is believed to be due to difference of wave-length, 
H. Becquerel, who, in 1880, made a special investigation 
of this branch of the subject, claims to have found that 



THE RELATIONS OF ELECTRICITY TO LIGHT. 289 

the rotations of different colored rays vary (very nearly) /// 
the inverse ratio of the squares of their wave-lengths. Thus 
taking the rotation produced by carbon bisulphide in 
green light as unity, that produced in red light is 0.6 
and in blue light 1.65. The wave-lengths assigned to 
each, in ten-millionths of an inch, being 211 for green 
light, 256 for red, and 196 for blue, if each number be 
divided by 211 and the quotients squared, the recip- 
rocals of the squares, expressed decimally, correspond 
approximately to the respective rotations given above, 
in accordance with Becquerel's law. 

Kiindt and Rontgen's Discoveries. — In 1879, Kiindt and 
Rontgen, with a 65-cell Bunsen battery, and electro- 
magnets wound with 2400 turns of wire, discovered the 
magnetic rotative force of air and other gaseous bodies, 
which Faraday with a 5-cell Grove battery failed to 
discover. They found that air, oxygen, nitrogen, car- 
bonic acid, coal-gas, ethyl, and marsh-gas, all rotate 
the ray in the direction of the magnetizing current, like 
water and carbon bisulphide; that the degree of rota- 
tion, which is very slight, varies greatly in differen* 
gases, and is proportional in each to the density of the 
gas; and that light, traversing the atmosphere in the 
plane of the magnetic meridian, is rotated, by the earth's 
magnetism, 1° for every 316 miles of air traversed. 

Becquerel, whose experiments were made a year later, 
found that the rotation of oxygen is opposite in direction 
to that of the other gases mentioned; such difference in 
observation being easily accounted for by the small de- 
gree of the observed rotation. 

Kiindt discovered, in 1884, that light transmitted 
through a film of iron, of such tenuity as to be trans- 
parent, is rotated in the direction of the magnetizing 
current, as in diamagnetic bodies. 

Kerr's Discoveries. — In 1875 I^'*- Kerr discovered that 



290 DYNAMIC ELECTRICITY AND MAGNETISM. 

light, polarized in a plane, when transmitted through a 
dielectric, at certain angles, under intense electric 
strain, suffers double refraction and is changed into 
that mode of polarization known as elliptical, in which 
the undulations occur in two planes crossing each other 
at right angles. 

For this purpose he used a rectangular prism of plate 
glass, in which holes were drilled at each end to within 
\ of an inch of each other at the center, into which were 
inserted the wire terminals of a powerful induction 
coil. A receptacle of similar shape, and of special con- 
struction, was also provided for experiments on various 
liquid dielectrics, as carbon bisulphide, benzol, paraffine 
oil, kerosene, oil of turpentine, and olive oil. 

The light, after passing through a polarizing crystal, 
was transmitted through the dielectric at right angles 
to the direction of the wires; the polarizer being turned 
as required to cause the plane of the polarized ray to 
form with this direction any angle desired; and the ray, 
thus transmitted, was received by the analyzer. 

This will be better understood if the dielectric be 
conceived as lying across this page, the direction of 
the wires being the same as that of the printed lines, 
and the ray, polarized in a plane, transmitted at right 
angles to the surface of the paper; the plane of the ray 
being turned so as to form an angle with the printed 
lines; as if a thin knife-blade, turned at an angle to the 
lines, were thrust through the paper. 

The ray being thus transmitted, and the analyzer 
turned so as to extinguish it, reappeared, on the passage 
of the current, when the electric strain reached a high 
degree of intensity; being brightest when the plane of 
the ray was at an angle of 45° to the direction of the wires 
— or electric strain — but becoming dimmer as the angle 
either increased or diminished; and being extinguished 



THE RELATIONS OF ELECTRICITY TO LIGHT. 29 1 

when the plane of the ray was either parallel to the 
direction of the electric strain or at right angles to it. 

Dr. Kerr's conclusion from these experiments is, that, 
in any given dielectric, the quantity of this optical effect — 
or intensity of electro-optic action — per unit of thickness of 
the dielectric^ varies directly as the square of the resultant 
electric force produced in the dielectric. 

In 1877 Dr. Kerr discovered that light reflected from 
the end of an electromagnetic pole having a polished 
surface, is rotated in a direction opposite to that of the 
magnetizing current, and hence in opposite directions 
by dissimilar poles. 

In order to concentrate the magnetic force on the 
polarized ray, he used a block of soft-iron which he 
called a " submagnet," having a rounded angle which 
was placed within ^ of an inch of one pole of a U elec- 
tromagnet. The ray, polarized in a plane either parallel 
or perpendicular to the plane of the angle of incidence, 
met the pole's surface in this narrow space, and was 
thence reflected to the analyzer, through which it passed 
when magnetized, being rotated as above, but by which 
it was extinguished when not magnetized. When the 
ray was polarized in a plane forming an oblique angle 
with the plane of the angle of incidence, the magnetism 
produced elliptic polarization, as in transmission through 
a dielectric under electric strain, and the ray could not be 
extinguished as before. The angle of incidence is that 
included between the incident ray and a perpendicular 
to the reflecting surface; its plane being known as the 
plane of incidence. 

Dr. Kerr also found that when polarized light is re- 
flected from the side of an electromagnet, the resulting 
rotation, except under certain conditions, is in the same 
direction as that of the magnetizing current. 

In this investigation he dispensed with the submagnet, 



2(^1 DYNAMIC ELECTRICITY AND MAGNETISM. 

and he used, for a reflector, the side of a soft-iron arma- 
ture, laid across the ends of the poles of a U electro- 
magnet. The ray, received through a slit in a screen, 
passed through the polarizer, and was reflected to the 
analyzer from a side of the armature perpendicular to 
that across the poles, in a plane at right angles to the 
magnet's plane. 

When the ray was polarized in a plane parallel to that 
of the angle of incidence, the rotation was in the same 
direction as that of the magnetizing current, for any 
angle of incidence; but when polarized in a plane per- 
pendicular to that of the angle of incidence, the rota- 
tion was in this direction only for angles of incidence 
between 75° and 80°, and in the opposite direction for 
angles between 75° and 30°. 

Effects of Double Reflection. — It has been observed that 
when light is polarized by reflection from a plane sur- 
face> a second reflection, in the opposite direction, from 
a parallel plane surface, at the same angle and in the 
same plane, annuls ordinary polarization but doubles 
magnetic polarization. Hence, with ordinary polariza- 
tion, an even number of such reflections annuls, while 
an odd number gives the same amount as a single re- 
flection: but, with magnetic polarization, the effect, 
under these conditions, is multiplied by the number of 
reflections. 

Summary. — The results of all these various observa- 
tions, in which are comprehended about all that is 
known of the relations of electricity to light, may be 
briefly summarized as follows: 

1. Light can be generated by electricity and electrici- 
ty can be generated by light. 

2. Polarized light, transmitted through a dielectric, 
has its plane of polarization rotated either by electro- 
magnetic force, by magnetic force alone, or by the force 



THE RELATIONS OF ELECTRICITY TO LIGHT. 293 

of an electric current alone, in the same direction as the 
current which produces, or would produce, the result- 
ing magnetism. 

3. Polarized light, reflected from the end of an elec- 
tromagnetic pole, has its plane of polarization rotated 
in a direction opposite to that of the magnetizing 
current, when polarized either parallel or perpendicular 
to the plane of incidence. But when reflected from the 
side of an electromagnetic armature, the rotation is, for 
nearly all positions of polarization, in the same direction 
as that of the magnetizing current. 

4. Light transmitted through a dielectric under elec- 
tric strain undergoes double refraction when polarized 
at an angle of 45° to the direction of the strain. 

5. Reflection which annuls ordinary polarization mul- 
tiplies magnetic polarization. 

Maxwell's Theory. — It has been already suggested 
that magnetism may be a mode of molecular or other 
motion having the phase of a vertical whorl around a 
central axis of propagation. This is the theory of Clerk 
Maxwell, in which he attributes magnetism to an un- 
dulatory motion of this kind in the ether. Applying 
this theory to the magnetic polarization of light, he 
conceives that the polarized ray, passing through the 
magnetic field, has its plane of polarization rotated into 
a new angle, in this magnetic whorl, in which it can 
pass through the analyzer, where it was before extin- 
guished. 

This theory certainly accounts in a very satisfactory 
manner for the opposite phases of rotation produced by 
opposite poles, and otherwise, under the various condi- 
tions of transmission and reflection which we have been 
considering. For if such a vertical whorl exists in the 
magnetic field, it is evident that the rotation of the 
polarized ray, in passing through it, would depend on 



294 DYNAMIC ELECTRICITY AND MAGNETISM. 

the angle between the plane of the ray and that of the 
whorl; so that the different phases observed to exist 
are just those which should result from such conditions. 

Molecular Theory. — It is not improbable that these 
phenomena may be due to modes of molecular motion, 
magnetic or electric, in the substance of the media, 
rather than to undulations of the hypothetical ether ; 
such a theory being as consistent with the various ef- 
'ects observed as that of the undulating ether. The 
rotation produced by reflection of the ray from a mag- 
net is no exception to this; the molecular motion of the 
reflecting surface producing the rotation, which is 
intensified by the passage of the ray through the mag- 
netic field having the air for its medium, to which the 
molecular motion of the magnet is communicated. 

Strain in the Media. — It is evident that the rotation 
of the ray, and the other effects observed, seem to result 
from magnetic or electric strain in the media rather 
than in the light itself, and that the effect on the light 
is secondary: still it is none the less evident that these 
effects are as truly modes of polarization as the polar- 
ization which occurs in the ordinary way in the crystal; 
the latter being, as we have seen, due to the peculiar 
crystalline structure, by which the undulations are all 
forced into the same plane, while, in the former, the 
structure of the media, solid, liquid, or gaseous, changed 
by magnetic or electric action, forces this plane into a 
new angle. 

The motion, given the media by Faraday, w^ould not 
disprove this, since it is probable that the magnetic 
action w^ould produce change of structure in the me- 
dium in each new position much more rapidly than the 
mechanical action could produce change of position; so 
that the direction of the strain would be the same as if 
the medium were stationary. 



THE RELATIONS OF ELECTRICITY TO LIGHT. 295 

Quincke attributes the double refraction obtained by 
Dr. Kerr to an electrostatic strain producing either 
expansion or contraction in the media according to the 
substance employed. Fontana noticed that the Leyden 
jar becomes slightly expanded when charged; an effect 
attributed by Volta, Priestley, and Duter to electric 
compression of the glass. 

Electric Lighting. — It has been shown that when the 
heat developed in a conductor by its resistance attains 
a sufficient degree of intensity light is produced; and 
on this principle, by the use of conductors of high re- 
sistance, we obtain the electric light, either as the 
result of incandescence or combustion. 

The Arc Light. — The electric light was discovered in 
1813 by Sir Humphry Davy, who obtained it by the 
passage of a current from 2000 voltaic cells through 
two rods of wood carbon, placed end to end, and, afte» 
the establishment of the current, slightly separated, 
producing a light of the most intense brilliancy having 
the form of an arc; hence the origin of the term voltaic 
arc or arc-light by which light, similarly produced, is 
designated, since it always assumes this form. 

It was subsequently produced with 40 Grove or 
Bunsen cells and rods made of carbon obtained from 
gas retorts, but remained as a laboratory experiment 
till brought into practical use, 60 years after its dis- 
covery, by the economical generation of electricity by 
the dynamo. 

Electric Candles. — One of the earliest and simplest 
methods of producing this light for practical use was by 
the electric candle; that of Jablochkoff, invented in 1872, 
being the first. It consisted of two carbon rods, each 
about 8^ inches in length and \ of an inch in diameter, 
imbedded in a cylinder composed chiefly of porcelain 



296 DYNAMIC ELECTRICITY AND MAGNETISM. 

clay, known as kaolin, at a distance apart of about -^ 
of an inch, and mounted vertically on a base. 

A dynamo current, passed up one rod and down the 
other, produced the arc light between them above. The 
kaolin being an insulator, the current was established 
between the rods by a carbon primer, connecting their 
upper ends, which was immediately consumed, and the 
current subsequently maintained by the incandescent 
carbon vapor. The rods burned slowly downward, con- 
suming the kaolin also, which increased the light by its 
incandescence. If a candle was accidentally extin- 
guished, a new primer was required to renew the current. 

The average duration of a candle was about \\ hours, 
but by using a group of 6, with automatic transfer of 
current, 9 hours continuous light could be obtained. 

The upward radiation with downward shadow, and 
:;he liability to accidental extinction, led to improve- 
ments, among which was the Jamin candle, constructed 
with 2 carbon rods, inclined toward each other at an 
angle, and fed downward by clock-work, making con- 
tact at the lower extremities for the establishment of 
the current, and having subsequent automatic separa- 
tion to form the arc. 

The sun lamp of Clerc and Bureau was another simi- 
lar device, in which the rods were fed downward by 
gravity, and maintained at the requisite angle and dis- 
tance 9.part by a block of marble or magnesia through 
which they passed. As they did not come into contact, 
a primer was necessary to establish the current, which 
was subsequently maintained by the conductivity which 
the block acquired by the heat, and which served also 
to prevent accidental extinguishing; the incandescence 
of the lime in the marble or magnesia increasing the light 
and modifying its color. The arc was from |^ an inch 
to 2 inches or more in length, while in the other can- 



THE RELATIONS OF ELECTRICITY TO LIGHT. 29/ 

dies its length was onl)^ -^ to \ of an inch; and the 
duration of this candle, with one pair of carbons, was 
about 10 hours. 

The Arc Lamp. — But all these devices were compara- 
tively short-lived, and were superseded by the arc lamp, 
now in general use, which, with various modifications, 
consists essentially of two carbon rods, as shown in Fig. 
93, maintained in a vertical position by automatic feed- 
ing devices controlled by the current which produces 
the light; being at first in contact, for the establishment 
of the current, but subsequently separated by the su- 
perior current strength thus acquired, to the normal 
distance required to form the arc; further permanent 
separation being prevented by the increased resistance 
which the arc acquires by increase of length, which 
weakens the current, causing the mutual approach of 
the carbons when the arc becomes abnormally long, or 
their contact for instantaneous relighting when acci- 
dentally extinguished. 

The Arc. — The arc thus formed consists of carbon 
vapor in union with oxygen. Its usual length varies 
from y^g- to \ of an inch, but for exceptionally strong 
lights it may be increased to f of an inch. Its electric 
resistance varies from \ an ohm to 100 ohms, and its 
illumination from 1000 to 2000 candle-power; its heat 
intensity being sufficient to volatilize the most refractory 
substances, not excepting the diamond. Its charac- 
teristic form is due to the difference of electric potential 
between it and the external air, by which it is attracted 
outward at the center while retaining its attachment to 
the carbons above and below; the potential difference on 
its opposite sides being unequal on account of its posi- 
tion being at the side of the central line of the carbons 
as shown below. 

When a direct current is employed, as shown by the 



298 DYNAMIC ELECTRICITY AND MAGNETISM. 

-j- and — signs in Fig. 93, a crater is formed in the upper 
carbon and a point on the lower, and the current pro- 
ducing the arc, following the path of least resistance, 
passes to the point of the lower carbon from the lowest 
projection on the irregular rim of this crater. As this 




Fig. 93. 

projection burns away the arc shifts to the next lowest 
point and thus travels continuously round the crater 
above, as if pivoted on the point of the lower carbon. 

The Crater and Point. — The formation of the crater is 
due, in part, to the checking of the current and conse- 
quent accumulation of energy above by the high resist- 
ance of the arc, causing increased consumption of car- 



THE RELATIONS OF ELECTRICITY TO LIGHT. 299 

bon. The exterior of both carbons is consumed more 
rapidly than the interior, consumption increasing to- 
ward the tips, producing a cone on each, the lower 
pointed and the upper truncated. There is also, prob- 
ably, a certain degree of electrolysis, producing excess of 
oxidation at the anode, or upper carbon, and correspond- 
ing diminution at the cathode; carbon vapor forming the 
electrolytic bath; the intensit3^of this action at the center, 
where the vapor is densest, producing the crater and 
point. In short arcs particles of carbon and fused im- 
purities are deposited on the cathode, forming the mush- 
room tip, sho\vn in Fig. 93, which is burnt off at the 
base and again renewed as the consum.ption proceeds. 

With the direct current, the positive carbon is con- 
sumed about twice as fast as the negative, but with the 
alternating current the consumption of both is equal, 
and both become pointed. 

The Heat and Light. — The heat is greatest in the car- 
bon vapor, and the light greatest in the incandescent 
carbon, 65^ of it being from the crater, the downward 
radiation from which is of special importance in the arc 
light, whose elevation for safety and convenience be- 
comes necessary in consequence of its intense brilliancy 
and the powerful currents required to produce it. 

Establishment of the Current. — The contact of the car- 
bons for the establishment of the current becomes 
necessary from the fact that a current sufficient to 
maintain the longest arc cannot pass through an air space 
of Yo^oT of an inch, while the momentary condensation 
of electric energy, and consequent high potential dif- 
ference produced betw^een the carbons previous to their 
separation, is sufficient to overcome the high resistance 
of the air film and cold carbon, and establish the arc, 
which is then maintained, through the reduced resist- 
ance, by the normal current. 



300 DYNAMIC ELECTRICITY AND MAGNETISM. 

The Carbons. — Carbon, originally used by Sir Hum- 
phry Davy in the discovery of the electric light, is still 
found to be the only substance suitable for its success- 
ful production; and it is of the highest importance that 
it should be pure and of homogeneous composition. 

Various carbonaceous substances have been employed 
for the production of the arc-light carbons, as coke, 
coal, charcoal, lampblack, graphite, and sugar; but pe- 
troleum coke, a residuum of the distillation of crude 
petroleum, has given the most satisfactory results. It is 
ground and then mixed with some hydrocarbon, as 
gas-house pitch, and after being thoroughly ground 
again, is molded in steel molds, heated and condensed 
by heavy pressure and the infiltration of hydrocarbon, 
md hardened and purified by repeated baking at vari- 
ous temperatures. 

The process involves numerous manipulations and 
equires great circumspection; the result being the 
production of carbons of remarkable purity and homo- 
geneousness. They are usually about 12 inches long, 
and vary in diameter from -^-^ to y^ of an inch, or more, 
in proportion to the current and candle-power required. 
They are beveled for concentration of the current, at 
the end intended for lighting, and usually copper-plated 
to within an inch of the point, for increase of conduc- 
tivity. 

Automatic Regulation. — The automatic regulation of 
the light is accomplished either by a train of clock-work 
or by a solenoid; both methods being in general use. 
The first is the oldest and was invented by Foucault, 
receiving various improvements in its earlier stages by 
Duboscq, Serrin, and Lontin; further improvements 
being subsequently added. 

In both methods the carbons are attached by sockets 
and binding-screws to brass rods supported v^^'tically, 



THE RELATIOA^S OF ELECTRICITY TO LIGHT. 3OI 



which, in the first method, are operated by the clock- 
work by means of electromagnets, through the coils of 
which the current passes. When the carbons are in 
contact or too close, the strong current through the 
magnet coil attracts the armature operating the clock- 
work and separating them, in opposition to the force of 
a spring, a weight, or an opposing current, which tends 
to bring them together; and as the current producing 
the separation becomes weakened by the increased re- 
sistance of the arc a balance between the opposing 
forces is obtained, by which the arc is maintained at its 
normal length. 

In the solenoid method, used by Siemens, Brush, and 
others, the upper carbon holder is lifted against the 
force of gravity by an armature to 
which it is attached, which moves ver- 
tically in the interior of a solenoid coil 
through which the current passes. 
As the armature is attracted upward, 
a clutch attached to it grips the edge 
of a loose washer, which being tilted 
grips and lifts the carbon holder which 
passes through it. 

Fig. 94 illustrates this and shows its 
application to the double carbon lamp, 
shown in Fig. 95. The clutch on 





Fig. 94. 



Fjg. 95. 



302 DYNAMIC ELECTRICITY AND MAGNETISM. 

the left being narrower than the one on the right> 
the left pair of carbons are kept apart by this simple 
device till the pair on the right are consumed, when 
the change of resistance instantly brings the left pair 
into contact, and the light is renewed. 

Hefner von Alteneck's Regulator. — The regulator of 
Hefner von Alteneck, of which Fig. 96 is an ideal illus- 




FlG. 96. 

tration, has an important application to the solenoid 
lamp and to arc lighting in general. 

The current from L to Li divides at /, the main branch 
going through the low resistance coil R\ and the lamp, 
as shown, while a shunt current of about i^ of the 
entire strength goes through the high resistance coil R 
and round the lamp. The armature ss is drawn down 
by the greater magnetism induced by the lower current, 
separating the carbons and establishing the arc. As 
the resistance of the arc increases with its length, the 
potential difference, or E. M. F., between L and L\ in- 
creases, and the strength of the lower current decreases 
in like proportion. But as the resistance in R remains 
constant, the strength of its current is increased by the 
increased E. M. F. in the same ratio as that in R\ is 
diminished by the increased resistance, tending to draw 



THE RELATIONS OF ELECTRICITY TO I.IGIJT. 303 

the armature ss upward by the increased magnetism in- 
duced and shorten the arc, which thus becomes adjusted 
to its normal length and a balance is maintained. 

These coils may be arranged in any convenient man- 
ner, as by winding in opposite directions, one outside 
the other; the shunt current thus opposing and par- 
tially neutralizing the magnetic effect of the main 
current, as in the Brush arc lamp. 

Series Distribution. — As currents of 10 to 15 amperes 
are usually required for arc lamps, the series method of 
distribution is found to be the most economical, and 
the only practical method; the entire current passing 
from lamp to lamp through a series often embracing 50 
or more, distributed over a large building, or area of a 
town. 

Automatic Cut-Out. — As any variation of resistance in 
a lamp affects every lamp in the series, regulators, con- 
structed on the principle of Hefner von Alteneck's, are 
required in the series system; also automatic short- 
circuiting apparatus for the exclusion of extinguislied 
lamps, without which the extinction of a single lamp 
would interrupt the current, causing the extinction of 
every lamp in the series. Such apparatus, in the Brush 
lamp, consists of an electromagnet wound with two 
coils, a fine wire coil on a closed circuit connected with 
the shunt, and a coarse wire coil on an open circuit 
connected with the magnet's armature. The ordinary 
shunt current does not induce sufficient magnetism to 
attract the armature, but the increased current, caused 
by the extinction of the lamp, is sufficient for this pur- 
pose; the attracted armature closing the coarse wire 
circuit, by which the full current is carried past the 
extinguished lamp. 

The Incandescent Lamp. — In the first attempts to pro- 
duce the electric light b}- incandescence exclusively, 



304 DYNAMIC ELECTRICITY AND MAGNETISM. 



platinum wire was employed and also iridium, but the 
superior advantages of carbon were soon demonstrated; 
consisting in its high electric resistance, 250 times as 
great as that of platinum, its infusibility at the highest 

temperature, and its greater 
illuminating power. But as it 
is volatilized at high tempera- 
tures in the presence of oxy- 
gen, its exclusion from the air 
became necessary, and this was 
accomplished by inclosure in a 
glass bulb in which a high 
vacuum was subsequently pro- 
duced by a mercury pump. 
Such are the general principles 
of construction of the incan- 
descent lamp as we now have 
it, as illustrated by Fig. 97. 

The Filament. — The carbon, 
prepared from a variety of dif- 
ferent substances, as bamboo, 
bass broom, cotton, linen, and 
silk, consists of filaments bent 
into any convenient form which 
will fit in the glass bulb. They 
are subjected to numerous ma- 
nipulations to give them the 
requisite hardness, tenacity, elasticity, homogeneousness, 
and durabilit3^ The principal steps are the forming; 
carbonizing by baking at a high temperature with ex- 
clusion of air; and '■'■ flashing,'' which consists in heating 
the carbonized filaments to incandescence by the elec- 
tric current or otherwise, in a bath of carbon vapor, the 
carbon from which is thus deposited on them, forming 
an even, dense, hard, homogeneous coating. The car- 




FiG. 97. 



THE RELATIONS OF ELECTRICITY TO LIGHT. 305 

bon of some filaments is entirely built up in this way on 
a base of fine platinum wire. There are also filaments 
made of hollow tubes for increase of surface. 

The average durability of a filament, in the 16 can- 
dle-power lamp, is from 600 to 1000 hours; the heating 
and cooling, molecular action, and general wastage, 
finally terminating in its rupture, requiring renewal of 
both filament and containing bulb. Its electric resist- 
ance, when heated to incandescence, is about half its 
cold resistance, ranging from 50 to 200 ohms, accord- 
ing to its length, cross-section, and composition. 

Filament and Lamp Attachment. — Each filament, when 
completed, is attached at both ends, as shown, to plati- 
num terminals sealed into the glass, after which the air 
is exhausted and the bulb hermetically sealed. 

Each bulb is then attached to a socket from which it 
can be easily removed for replacement; in which is a 
device, operating with springs, for closing or opening 
the circuit by turning the insulating handle shown, by 
which the current is passed through the filament or ex- 
cluded from it for lighting or extinguishing the lamp. 

Position and Current. — The position of this lamp when 
in use is entirely a m.atter of convenience, as its illumi- 
nation seldom exceeds 16 candle-power, and its current 
J to f of an ampere. The current may be either direct 
or alternating according to the system of lighting, each 
system having numerous distinctive features. 

Parallel Distribution. — The large number of lamps re- 
quired on an incandescent lighting circuit and the 
small current required for each makes the parallel sys- 
tem of distribution the most economical and practical. 
This system is illustrated by Fig. 98, in which are rep- 
resented two heavy copper mains issuing from the 
dynamo, between which the lamps are mounted on fine 
wire connections. 



3o6 DYNAMIC ELECTRICITY AND MAGNETISM. 



These mains may extend to any required distance 
through a building, or through streets, 
with branch mains extendinginto the build- 
ings; but when the direct current is em- 
ployed, they must be of sufficient size to 
reduce the resistance to a required mini- 
mum. A copper conductor capable of 
carrying a current sufficient to feed 5000 
16 candle-power lamps at a mean distance 
of 4000 feet from the dynamo would require 
a cross-section of 12.57 square inches, the 
size being proportionally reduced as the 
line branches into parallel circuits, while 
wire of No. 14 to 16 gauge is large enough 
for the lamp connections. 

If a circuit, like that shown in Fig. 98, 
have a resistance, including that of the 
dynamo, of i ohm, and each filament a hot 
resistance of 199 ohms, and the dynamo an 
effective E. M. F. of 100 volts, then, if a 
100 volts I 
2 



Fig. 98. 
single lamp be lighted, it has 



= — an am- 



200 ohms 

pere current. But if two lamps be lighted, the current 
has two paths instead of one between the mains, which 
is the same, in effect, as doubling the cross-section of 
the filament and thus halving its resistance ; which gives 



199^ 



+ 



2^ 

2 



20\R 



= looji? ; then, if the fraction be 



neglected, ^ — iC for the 2 lamps, i an ampere to 

each, as before. 

For any small number of lamps the resistance varies 
inversely and the entire current directly as the number 
lighted, and the current per lamp remains practically 
constant, as shown, being equally divided among the 



THE RELATIONS OF ELECTRICITY TO LIGHT. 307 

entire number lighted. But as the resistance of the 
dynamo and circuit remains constant while that of the 
lamp filaments varies, it is evident that in the lighting 
of any considerable number of lamps the fraction, neg- 
lected above, would make a sensible difference in the 
ratio of resistance to E. M. F. Suppose that 100 were 
lighted, then the entire filament resistance would be 

— — = i.^gR, and, adding in the i ohm constant re- 
sistance, we have 2.99 ohms as the entire resistance; 

hence = 33t*AC which, divided among the 100 

2. 99-/l 

lamps, gives about ^ of an ampere per lamp, instead of | 

an ampere, with only one or two lighted. 

There is also a certain amount of current wastage, 
making an entire current variation of 15^ to 20^, which 
must be provided for in order to maintain constancy of 
current and illumination. This, in the direct current 
system, is done by the introduction of resistance coils 
into the circuit, by which the current can be varied by 
variation of the resistance, and in the alternating cur- 
rent system by a direct variation of current in the con- 
verter. 

Hence when the indicator at the station shows a 
variation of current below or above the normal, by the 
lighting or extinguishing of any considerable number 
of lamps, the attendant makes the necessary correction 
by moving a switch either in the resistance box or con- 
verter according to the system of lighting employed. 

Multiple Series and Series Multiple. — A number of 
short series of lamps may take the place of single lamps 
on a parallel circuit, producing what is termed a "mul- 
tiple series" installation; or a number of groups with 
lamps in parallel in each m.ay be placed in series, pro- 
ducing what is termed the " series multiple" installation. 



308 DYNAMIC ELECTRICITY AND MAGNETISM. 



Three-Wire System. — In the Edison three-wire system, 
illustrated by Fig. 99, two parallel circuits with two 
dynamos are combined, the dynamos be- 
ing connected together in series as shown ; 
and a single central main, attached to the 
short connector which joins them, takes 
the place of the two interior mains, and 
equalizes the current through the lamps, 
in the following manner. When an equal 
number of lamps is lighted on each cir- 
cuit, the resistance between the circuits 
being equally balanced, the entire current 
flows across through the several pairs of 
lamps in series between the two external 
mains. But the lighting of a greater 
number on one circuit than on the other 
reduces the resistance and increases the 
current in that circuit; and this surplus 
current flows through the central main; 
^ I. yST" in a negative sense if the increase is in the 
left-hand circuit, but in a positive sense 
if the increase is in the right-hand circuit. 
Three mains are thus enabled to do the work for 
which four are usually required. But a further reduc- 
tion in the required amount of conducting metal results 
from the fact that this amount is found to vary in- 
versely as the square of the required E. M. F., which 
being doubled by joining the two dynamos in series, the 
cross-section of each main should be reduced to \ the 
usual amount, its length remaining the same if there 
were no change of filament resistance. But the joining 
of each pair of lamps in series increases the filament re- 
sistance to four times the amount of that of each pair 
joined in parallel, the current traversing twice the fila- 
ment length with half the cross-section. Hence the 




THE RELATIONS OF ELECTRICITY TO LIGHT. 3O9 

ratio of resistance to E. M. F. would be doubled if mains 
of only \ the usual size were employed; therefore, to 
maintain constancy of current, mains of \ the cross- 
section and same length would be required; that is, 3 
mains, each \ the usual size, or f the usual amount of 
copper, if each main were required to carry a full cur- 
rent. But, as the central main carries only the required 
surplus of current, its cross-section can be reduced 
about 13^^ below that of the other two. 



310 DYNAMIC ELECTRICITY AND MAGNETISM. 



CHAPTER Xn. 
THE ELECTRIC TELEGRAPH. 

Early History. — The experimental stage of tne elec- 
tric telegraph extends back to the middle of the last 
century; static electricity having been first employed 
for the transmission of signals; a plan for alphabetic 
signaling by this means being described in Scot's Mag- 
azine for 1753. Lesage constructed the first electric 
telegraph, in 1774, at Geneva; in which he employed 24 
wires, each connected with a separate pith-ball electro- 
scope, representing a letter of the alphabet. Similar 
methods were employed later, by Lomond in 1787, and 
Ronalds in 1816. Reusser, in 1774, suggested the illu- 
mination of letters made with metal spangles on glass 
plates, as an improvement on the pith-ball method of 
Lesage. 

Sommering, in 1808, first employed voltaic electricity 
for telegraphing, using 35 water voltameters, each con- 
nected with separate wires and giving separate signals: 
and similar methods were subsequently tried by Coxe, 
Smith, Bain, and others. 

Ampere, in 1820, proposed to employ 24 galvanometer 
needles, each connected with a separate wire. Schilling, 
in 1832, and Weber and Gauss, in 1833, employed a 
single needle, indicating alphabetic signals by right and 
left deflections. Steinhill subsequently developed this 
system, employing two needles, one for the positive and 
the other for the negative current, both deflected in the 
same direction; alphabetic signals being given by bells 



THE ELECTRIC TElEGRAPI/. 311 

Struck by the needles, and also by dots made with ink 
on a moving strip of paper, as well as by observation of 
the movements by the eye. 

Steinhill, while constructing a telegraph line at Mu- 
nich, in 1838, made the very important discovery that 
the current could be carried by a single wire, and the 
earth employed for the return circuit by making con- 
nection with it at the terminals of the line; from which 
he inferred that the earth took the place of the return 
wire as a conductor; but subsequent experiments seem 
to prove that the earth, in this case, is to be regarded as 
an electric reservoir, giving and receiving electric energy, 
rather than as a conductor. 

Cook and Wheatstone, in 1837, introduced the needle 
telegraph, as it was designated, into England, and con- 
structed, on the London and Birmingham Railway, the 
first line ever employed for commercial use. It con- 
sisted of five underground wires connected with five 
separate needles; a system which they subsequently 
modified, employing two wires connected with two 
needles in one method, and a single wire and needle in 
another method. The signal for the transmission of a 
message was given by a bell rung by an electromagnet. 

In 1831 Henry transmitted signals by sounds pro- 
duced by the movements of the armature of an electro- 
magnet; and Morse, in 1835, invented a telegraph oper- 
ated in a similar manner, in which alphabetical signals, 
consisting of lines and dots, were made on a moving 
strip of paper, first by a pencil, but subsequently by a 
steel point which embossed them on a grooved roller 
over which the paper was moved by clock-work oper- 
ated by a weight. 

Morse constructed the first commercial telegraph line 
in the United States, between Washington and Balti- 
more, and sent the first message over it May 27, 1844. 



^12 DYNAMIC ELECTRICITY AND MAGNETISM. 

This line was mounted on wooden poles and consisted 
of two iron wires, the practicability of employing the 
earth for the return circuit being then imperfectly 
understood. 

The American Morse Code. — The original Morse code 
for letters, numerals, and punctuation, now employed 
in the United States and Canada, is as follows: 

A B C D E F G HI 

J K L M N P 



Q R S T U V W X 

Y Z &= Period Semicolon Comma 

Exclamation Interrogation Paragraph Parenthesis Italics 
I 2 3 4 5 6 



It will be noticed that this code consists of long 
dashes, short dashes, dots, and spaces; Z for instance 
being indicated by a long dash, 7" by a short dash, J? 
by three dots with space between the first and second, 
C by three dots with space between the second and 
third. Hence the number and relative positions of 
these four elements constitute the distinction between 
the different characters; the spaces having equal sig- 
nificance with the dashes and dots. 

The International Morse Code. — The Morse code has 
been found so well adapted to telegraphing, that it 
has superseded all others for this purpose, and come 
into general use throughout the world. But on its in- 
troduction into Europe some changes were necessary tc 



THE ELECTRIC TELEGRAPH, 



13 



adapt it to the various languages, and also to remedy 
defects which had been developed by its practical use 
in America. This led to the adoption, by a telegraphic 
convention assembled at Vienna in 1851, of the inter- 
national Morse code, now employed in all countries ex- 
cept the United States and Canada. In this code long 
spaces between the elements of a letter are eliminated, 
as they are liable to be misunderstood for the spaces 
between letters; each numeral is represented by five 
elements, and each punctuation mark by six. The dif- 
ferences between this code and the American are as fol- 
lows: 



c 


F 


J 


L 





p 


Q 


R 


X 


V 


z 


Ch 


A 





u 



E N Period 

Exclamation Interrogation Apostrophe 

Parenthesis i 2 

567 S 



Comma 



Hyphen 

3 



As it was soon found that messages could be read 
more easily and rapidly by the click of the instrument 
than by the record on the paper, the dots, dashes, and 
spaces came to indicate sounds and pauses, and the 
registering instruments were replaced by sounders in 
all the principal offices. 



314 DYNAMIC ELECTRICITY AND MAGNETISM. 

Simple Line Equipment. — The principal apparatus re- 
quired for the equipment of a simple Morse telegraph 
line are a battery^ signal key, sounder or register, and relay; 
all of which must be duplicated at each end of the line; 
the duplication of the battery, on such a line, being de- 
sirable though not always strictly necessary. A light- 
ning arrester, ground switch, and cut-out are also required. 

The Battery. — The principal requirements of the bat- 
tery are strength and constancy, and any good battery 
fulfilling these conditions can be employed. The gravity 
battery is one of the best and cheapest, and hence is 
extensively used for this purpose. It requires com- 
paratively little care, is free from noxious fumes, and 
not liable to polarize. 

The Key. — The key, one form of which is shown in 
Fig. IOC, is a lever of steel or brass, so mounted as to 




Fig. ioo. 
have a vertical movement, limited by two set-screws 
which can be adjusted to any required range of motion; 
the upward movement being produced by a spring con- 
nected with one of the screws, and the downward by 
pressure on the hard-rubber knob at the left, which 



THE ELECTRIC TELEGRAPH. 



315 



closes the circuit by bringing a little projection under- 
neath into contact with an anvil attached to the left 
hand bolt; the points of contact being faced with plati- 
num to prevent fusion by the extra current at break. 

This bolt and anvii are insulated from the support- 
ing frame, while the bolt at the right is connected with 
it, and both are connected with the terminals of the 
electric circuit. When the key is not in use the circuit 
is closed by a lever pushed under a metal projection at- 
tached to the anvil. 

The Register. — A simple form of the embossing register 
is shown in Fig. loi. The armature of an electromag- 
net ilf is attached to the bent lever Z, pivoted at d so 




Fig. ioi. 

as to have a vertical movement limited by the adjust- 
able screw m and stop underneath. A steel point/, 
attached by an adjustable screw to the bent end of 
the lever, makes contact in a little groove with the 
roller r, and embosses the message on a strip of paper 
carried between the rollers, which are operated by 
clock-work impelled by a weight attached to a cord 
wound on the drum W, and controlled by the brake a. 
The electromagnet is connected with the line by 
binding-posts, one of which is shown at j, and when the 



3l6 DYNAMIC ELECTRICITY AND MAGNETISM. 

current, transmitted from the distant station, attracts 
the armature, the lever L is drawn down against the 
force of the spiral spring «, bringing the point into con- 
tact with the paper, and registering the message as de- 
scribed. 

Double embossing registers, operated on the same 
principle, are now in common use, by which two sepa- 
rate messages can be registered in parallel lines on the 
same strip of paper, or one message only, on a single 
line, as required. Inking registers, both double and 
single, are also in common use, and are generally pre- 
ferred to the embossing instruments. The clock-work, 
in all the new registers, is operated by a spring. 

The Sounder. — One of the best known forms of the 
sounder is shown in Fig. 102. A bent lever, having a 




Fig. 102. 

vertical and a horizontal arm, is pivoted on an arched 
support as shown, the vertical arm being concealed by 
the support. The horizontal arm has a vertical move- 
ment between the poles of an electromagnet, limited by 
the adjustable set-screws shown above on the left; and 
is held in contact with the upper screw, when the cip 



THE ELECTRIC TELEGRAPH. 31/ 

cuit is open, by a retractile spring connecting the lower 
end of the vertical arm with an adjustable screw which 
passes through the supporting post on the left. 

The instrument is connected with the line by the 
binding-posts on the right, and when a current is sent 
through the coils of the magnet the attraction of the 
armature brings down the lever, the point of the screw 
striking with a sharp click on the curved brass sounding 
piece. When the current ceases the spring brings the 
lever up with a light click against the screw above; and 
by means of these two clicks, signals indicating the dots, 
dashes, and spaces are distinguished. The sharp click 
indicates the beginning of a dot or dash, and the light 
click its termination; a pause following a sharp click 
indicates a dash, and a pause following a light click in- 
dicates a space. 

The screws can be adjusted to any required range of 
motion, both in the sounder and register; and the arma- 
ture, in both instruments, is kept out of contact with the 
magnet poles, to prevent magnetic adhesion. 

The Relay. — On short, well-insulated lines, not ex- 
ceeding 20 or 30 miles in length, the sounder, or register, 
if its resistance is not too high, can be operated by the 
line current; but, on longer lines, resistance and im- 
perfect insulation usually weaken this current too much 
for direct action; but by the aid of a relay it can per- 
form this work indirectly through the agency of a local 
battery current. 

A common form of the relay is shown in Fig. 103. 
An electromagnet supported in a horizontal position by 
an adjustable screw, on the right, and a curved stand- 
ard, on the left, has its armature attached to a vertical 
lever, pivoted below, but having a horizontal movement 
above limited by two screws by which its range of mo- 
tion can be adjusted. A retractile spring holds it 



3l8 DYNAMIC ELECTRICITY AND MAGNETISM. 

against the poiiit of the left hand screw when the cir- 
cuit is open, while a weaker spring below tends to force 
it in the opposite direction; the tension of the upper 
spring being capable of adjustment as shown. 

The two binding-posts on the left are connected re- 
spectively with the curved standard and lever support, 
and, exteriorly, with a local battery which embraces in 




Fig. 103. 



its circuit the sounder or register; and the electromag- 
net is connected with the line by the two binding-posts 
on the right. Hence, when the line current passes 
through the magnet coils, the armature is attracted and 
the local circuit closed by contact between the platinum 
points attached to the lever and right hand screw, and 
the sounder or register operated by the local current. 

The distance between the magnet and its armature 
can be adjusted by the supporting screw on the right, 
which moves the magnet through the openings in the 
curved support on the left. Hence the adjustment by 
this means and that of the retractile spring and upper 
screws can be adapted to any current which may be 
sent over the line. 



THE ELECTRIC TELEGRAPH. 319 

Cut-Out, Ground Switch, and Lightning Arrester. — These 
three instruments, of which there are several different 
forms, are employed separately, or may all be combined 
in one. A common form of the latter kind is shown in Fig. 
104. Three brass plates, with binding-posts attached, are 




Fig. 104. 

mounted on an insulating block, the central plate hav- 
ing a row of points on each side. This block is attached 
to the wall in any convenient position, the end plates 
connected with the terminals of the line, and the office 
instruments placed in circuit between them, and the 
central plate connected with the earth. 

When a brass plug is inserted between the end plates, 
as shown, the office instruments are cut out of the 
circuit, but when the plug is removed the current must 
evidently pass through the instruments. If, under the 
latter arrangement, lightning should strike anywhere 
on the line, its high potential would cause its current 
to pass to the earth by way of the points and central 
plate, instead of taking the longer route through the 
instruments. 

When the line connections, at a way station, are inter- 
rupted, the direction in which the interruption has 
occurred may be ascertained, and current from the 



320 DYNAMIC ELECTRICITY AND MAGNETISM. 

opposite direction obtained by making connection with 
the earth. This is done by inserting the plug between 
the central plate and one of the end plates. If the inter- 
ruption were on the right and the plug should happen 
to be first inserted on the left, no current would be ob- 
tained ; if then the plug were inserted on the right, the 
instruments would be placed in circuit between the 
earth and the uninterrupted connection on the left and 
current obtained; and in like manner connection could 
be established on the right, if the interruption were 
found to be on the left. 

The cut out should always be closed during a thunder- 
storm or the absence of the operator, to prevent acci- 
dents to the instruments. 

Line Construction. — The ordinary telegraph line is con- 
structed with No. 6-7 iron wire, B. and S. gauge, but for 
short lines No. 8-9 wire can be used. It is coated with 
zinc to prevent oxidation, and mounted on wooden 
poles, provided with supporting cross-arms to which it 
is attached by glass insulators; a large number of 
parallel wires, arranged in tiers, being often mounted 
on the same poles. 

In cities where air lines are prohibited, the wires, 
coated with insulating material and combined in cables, 
are laid underground; the cables being inclosed in lead 
pipes, or otherwise protected against moisture and 
abrasion, and often placed in conduits so as to be acces- 
sible without disturbance of the pavement. 

Joints are made, where required, by twisting the 
wires firmly together, and then soldering them to insure 
perfect electric connection and prevent its interruption 
by oxidation ; but electric welding, now coming into use, 
is preferable for this purpose. Where wires pass through 
walls, for office connections, they require to be well 
insulated by hard-rubber tubes. 



THE ELECTRIC TELEGRAPH. 



321 



Station Arrangement. — The arrangement of the instru^ 
ments and connections of a terminal station are shown 
in Fig. 105. The line is connected with the earth at G^ 
the wire being soldered to a mass of buried metal for 
good connection, or to water or gas pipes, where they 
are available for this purpose. It is connected with the 
main battery shown at E^ which has its positive pole 
connected to the earth and its negative to the line; this 




arrangement being reversed at the opposite end of the 
line; hence the line current, at this station, always flows 
towards the battery, and at the opposite terminal sta- 
tion, fro7n the battery, while the earth current, at each 
station, flows in the opposite direction. The instruments 
and battery may be cut out, when necessary, by a switch 
or plug connection, at X, with the ground wire shown 
at the left. 

When a message is being received, the circuit is closed 
through the key K^ and the line current, entering 
through the lightning arrester at X, traverses the relay 



322 DYNAMIC ELECTRICITY AND MAGNETISM. 

M by the binding-posts i and 2, and goes through K 
and battery E to the earth at G. The circuit of the 
local battery E\ being closed by the relay, its current 
passes from the positive pole, by the binding-posts 3 
and 4 and platinum points, through the sounder 6* and 
thence to the negative pole of E' . 

When a message is being sent, the circuit is opened 
at X, and the instruments at the distant station, 
traversed by the outflowing current, respond to the 
manipulations of the key at the home station, where 
the instruments are traversed by the inflowing current 
as before. The same conditions, vi^ith reversal in the 
direction of the current, occur at the opposite end of 
the line. 

It is important that the instruments should always 
be in circuit during business hours, as a station is liable 
to be called at any moment; and their constant click is 
a notice to the operator that his connections are right 
and the line in working order. All the messages and 
station calls are therefore heard at every station on the 
line, but responded to only at the station called. 

The arrangement of a way station is the same as that 
of a terminal station except that only the local battery 
is required; the current from the main batteries enter- 
ing by one branch of the line, traversing the relay and 
key and operating the sounder by the local battery, and 
leaving by the other branch of the line. Messages can 
therefore be received or sent in either direction, the 
current being positive to all the stations on one side and 
negative to all those on the other. 

The current is derived from both terminal batteries, 
which may be regarded as a single battery, with the 
instruments interposed between its poles in one branch, 
and the earth in the other. Hence if either battery is 



THE ELECTRIC TELEGRAPH. 



323 




OOO^DO 



^^ooooo ' 





000000 

"^ la's 




00000® 






324 DYXAMIC ELECTKiClTV AND MaGXETIsM. 

cut out by a ground switch, either at a terminal or way 
station, the current is proportionally reduced. 

Switch Board. — When a number of different lines have 
connections through the same office a switch-board, 
like that represented by Fig. io6, is required. The 
vertical brass bars represent line connections and the 
rows of brass disks, battery connections. The disks in 
each horizontal row are electrically connected together 
at the back, but insulated from the other rows; and each 
row, except the lowest, connected with a separate bat- 
tery, the lowest being connected with the earth. 

By the apparatus known as a spring-jack the instru- 
ments may be connected with the line as shown at 
SJ'"\ a wedge W having brass plates on its opposite 
sides, insulated from each other, but connected with the 
terminals of the instrument circuit, being inserted be- 
tween a pair of springs, which close the circuit again 
when the wedge is withdrawn. 

The bars B and B^ being thus connected with the 
line, the insertion of a plug at H puts battery MB^ in 
connection with the line L by the vertical bar B\ and, 
in like manner, battery MB is connected with Z' by the 
plug 7^ and bar ^'. Any line may be connected with 
the earth for testing by inserting a plug between its 
bar and the lowest row of disks. Thus the insertion of 
a plug at M gives the line connected with B^ an earth 
connection. 

Repeaters. — As the distance to which messages can be 
transmitted is limited, even with the aid of the relay, it 
becomes necessary to have them automatically repeated 
by a special' apparatus which employs local batteries. 
By this means stations four or five thousand miles apart 
can hold communication with almost the same facility 
as those on short lines. Press despatches can also, in 



THE ELECTRIC TELEGRAPH. 325 

this way, be received simultaneously by all the princi- 
pal stations on a line. 

Before the introduction of this method the message 
had to be repeated by the operator, involving great de- 
.ay and liability to errors. 

Two sets of instruments are required for repeating, 
each connected with a separate branch of the line, and 
including a relay, sounder, and also two or more local 
batteries to each set. 

The Button Repeater. — The button repeater^ invented 
by Wood in 1846, and still employed to a limited extent, 
was one of the first in use. It consists, as improved, of 
a button switch placed between the two sets of instru- 
ments, having double contacts on each side, one pair of 
which may be closed when the other pair is opened, or 
both pairs opened as required. Each set of instruments 
is in circuit between each pair of contacts, and when 
either pair is closed the two branches of the line are 
connected through both sets of instruments, and each 
connected also with a separate main local battery. 

When a message is to be repeated, the operator at 
the repeating station, on being notified, closes the 
switch contacts through the branch of the line wishing 
to transmit, giving it a closed connection to the earth 
through the main local battery of the repeater, with 
which it is connected; which enables the operator at 
the sending station to repeat into the opposite branch 
through the sounder connected with his branch, which 
also acts as a transmitter. When a message from the 
opposite direction is to be repeated, the connections 
are reversed by reversal of the switch. 

In this repeater both sets of instruments respond to 
the manipulations of the sending operator's key, but 
the closed switch contacts prevent any break in the 
through connections. When, however, the switch is 



326 DYNAMIC ELECTRICITY AND MAGNETISM. 

disconnected from the contacts on both sides, the 
through connection being opened, each branch of the 
line becomes independent of the other, and terminates 
in its main local battery at the repeating station. Mes- 
sages can then be sent or received by each set of in- 
struments, separately, by connecting them with local 
keys, or through connections, independent of the re-« 
peating apparatus, made with a single set. " 

The Milliken Repeater. — The annoyance and delay 
occasioned by the absence of operators at repeating 




Fig. 107. 

stations led to the invention of repeaters which can be 
reversed from distant stations by the current, automatic- 
ally. The Milliken repeater, shown in Fig. 107, is one 
of the best known of these. It consists of the ordinary 
lelay, shown at the right, by which the sounder is 



THE ELECTRIC TELEGRAPH. 32/ 

Operated in the usual manner, and an extra magnet, 
mounted at the left, whose armature is arranged to 
close the circuit automaticalh^ and keep it closed, 
through one branch of the line, by the aid of connected 
apparatus, during the repeating of a message by a sim- 
ilar companion instrument connected with the other 
branch. The upper retractile spring has greater ten- 
sion than the lower, so that when the attraction of the 
extra magnet ceases, its armature is pulled back, bring- 
ing an insulated stop against the armature of the relay, 
as shown, and closing the local circuit through the con- 
nected sounder. 

Repeater Connections.— The arrangement and connec- 
tions of the Milliken repeater, at a station, are shown in 
Fig. io8. T and V are sounders, used also as trans- 
mitters, and connected respectively, at Y and F', with 
the extra magnets ExM and ExM' of the opposite re- 
peaters through the circuits of the extra local batteries 
XL and XL' . The levers of T and T' are furnished 
with continuity-preserving springs s and s\ insulated 
from them, which make contact with stops above, when 
the levers are attracted, and close the circuits of the 
main local batteries MB and MB' respectively; the 
lever projections, at jc and x' ^ limiting the upward move- 
ment of the springs, when the attraction ceases, and 
breaking the contact. In this way each line circuit is 
closed by a spring through its main local battery before 
the closing of the circuit at the opposite end of the 
lever through the extra local battery, and remains closed 
till after the latter circuit is opened. Thus when the 
lever of 7" is attracted, the circuit of MB is closed at s 
before that of XL is closed at F, and remains closed till 
after the contact at Y is again opened. The dark space 
at the mounting of these springs shows hard-rubber in- 



328 DYNAMIC ELECTRICITY AND MAGNETISM. 




THE ELECTRIC TELEGRAPH. 329 

sulation, which is similarly indicated at various other 
points. 

The main local battery MB is connected with the 
eastern line through relay R\ and MB' with the western 
line through R\ each battery being connected with the 
earth as shown. 

When a message from the east is to be repeated to 
the west, the eastern operator opens the circuit through 
his key, and the operator at the western terminal station, 
finding this to be the case, closes the circuit through his 
key; hence there is current in the western line through 
relay i?, but none in the eastern line through R' \ arma- 
ture B is therefore attracted, closing the local circuit 
EE at C\ and transmitter 7", being attracted, first closes 
the eastern line at i", through battery MB, and then the 
circuit of XL at Y. The magnet ExM' therefore at- 
tracts its armature, allowing the spring S'" to open the 
local circuit E'E' atC: this releases T' , opens the cir- 
cuit of XL' at F', and then the western line at s' , break- 
ing connection with battery MB'. This break stops the 
current on the western line, demagnetizing relay R\ but 
R's armature is still held closed by the superior force 
of the spring .S* over that of S" \ hence the connections 
on the left remain closed while the instruments on the 
right respond to the manipulations of the eastern oper- 
ator's key, enabling him to repeat into the western line. 

In a similar manner the western operator, by opening 
the circuit through his key while that through. the key 
at the eastern terminal station is closed, can produce 
automatic reversal of the connections at the repeating 
station, and repeat into the eastern line. 

Duplex Telegraphy. — The simultaneous transmission 
of messages in opposite directions on the same wire 
occupied the attention of various inventors from 1852 
to 1872. The first suggestion of the practicability of 



330 DYNAMIC ELECTRICITY AND MAGNETISM. 

this method was made by Moses Farmer, an American, 
in 1852, and the first invention of the kind by Gintl, an 
Austrian, in 1853. Gintl's invention not proving suf- 
ficiently practical, improvements on it were made, in 
1854, by Frischen, Siemens, and Halske, and as the re- 
sult of their labors the duplex system was first put in 
successful, practical operation, in 1855, between Munich 
and Vienna, and subsequently, in the same year, between 
Vienna and Trieste. 

Preece, Nystrom, Maron, and other European invent- 
ors made various valuable contributions to the duplex 
system between 1855 and 1863, but it was almost un- 
known in America till 1868, when Stearns began a series 
of experiments based on the European methods which 
resulted, in 1872, in the practical adoption of his system 
in the United States. 

The Stearns Duplex. — The construction and operation 
of this system is shown in Fig. 109, in which the con- 
nections at terminal stations on the right and left are 
represented. R and R' are differential relays connected 
with sounders not shown, in each of which the two bob- 
bins are each oppositely wound, as shown, so that cur- 
rents of equal strength in each would neutralize each 
other's magnetic effect on the cores, while a current in 
either branch of the circuit alone, or of greater strength 
in one branch than in the other, would magnetize the 
cores. Rh and Rh' are rheostats whose resistance is so 
adjusted that the resistance from the central point a or 
a' of either relay through the rheostat to the earth is just 
equal to the resistance in the opposite direction through 
the line. T and T' are transmitters (not sounders) 
operated by the keys and small local batteries shown, 
and whose levers A and A' have the continuity preserv- 
ing springs, z and z\ already described. MB and MB' 
are the main batteries, connected with the line through 



THE ELECTRIC TELEGRAPH, 



331 



@ l|i|llililif 




e — '1 



;^}2 DYNAMIC ELECTRICITY AND MAGNETISM, 

z and z\ and also connected with the earth. .SC and 
SC are resistance-coils, also connected with the line 
and earth, whose resistance is made equal to that of the 
batteries MB and MB' respectively, so that the line 
resistance to the earth shall be the same whether the 
connection is through the coil or through the battery. 
Cand Care condensers adjusted to absorb a charge 
equal to the static charge absorbed by the line, and 
return an opposing current equal to the return current 
produced by that charge, and thus neutralize it and 
prevent a false signal. 

When a message is to be sent from the station on the 
left, the depression of key K closes the local circuit 
through the magnet of transmitter T\ and the conse- 
quent attraction of lever A closes the circuit of battery 
MB^ through spring z^ and opens the ground circuit 
through SC at x by the depression of the spring as 
shown. 

A current from MB therefore flows through both 
branches of relay R, and the resistance being the same 
in each, divides equally at ^, one half going to the line 
through o^ fly and the other half to the ground through 
;;?,/, and rheostat Bh^ except the portion absorbed by 
condenser C. Hence, no magnetic effect being produced 
in i?, its armature is not attracted; but the line current 
entering relay R' passesonly through branch n\o', hence 
the core of R' is magnetized and its armaturs attracted, 
producing a down click in the connected sounder. 

Now let key A"' be also depressed, closing the circuit 
of MB' through spring z' , and opening the ground 
circuit through SC at x'\ a current then flows to MB' 
from MB through branch n', o' ^ of R' \ this doubles the 
current through «', <?', and hence the armature of R' is 
still attracted and kept closed. But it also doubles the 
current through branch <?,;/, of relay R, magnetizing its 



THE ELECTRIC TELEGRAPH, 333 

core, and hence attracting its armature and producing 
a down click in the connected sounder. 

Now let key X be opened, and the circuit of battery 
MB through spring z being thus opened, its current 
ceases; but the ground circuit through SC to battery 
MB\ being closed through x before the circuit through 
z is opened, i? is still magnetized by J/!^"s current, and 
hence its armature remains closed. But MB's current 
being removed from n\ o\ of relay R\ and the current of 
MB' flowing equally through m\p\ and o\ n\ B' is de- 
magnetized and its armature released, producing an up 
click in its connected sounder. 

Now let key K' be opened also, and the current of 
MB' ceases, demagnetizing relay B, whose armature 
being thus released, an up click is produced in its con- 
nected sounder also. 

Each sounder therefore responds only to the manipu- 
lations of the key at the opposite station and is unaf- 
fected by the manipulations of the home key, and hence 
messages are transmitted simultaneously in opposite 
directions over the same wire with the same facility as 
in single transmission. But it is evidently essential to 
the successful operation of this system that the adjust- 
ment of the resistances in both sets of instruments shall 
be carefully maintained, so as to produce currents of 
equal strength in both branches of each relay. Each of 
these currents has the same strength as if the current 
had not been divided, the E. M. F. and resistance re- 
maining the same in each branch. 

The Polar Duplex. — The polar duplex system, as im- 
proved, originated from the various inventions which 
culminated in the invention of the improved quadruplex 
system. Its two principal instruments are the pole- 
changer 3.n6. polarized differ€7itial relay, the latter the in- 
vention of Dr. Siemens. 



334 DYNAMIC ELECTRICITY AND MAGNETISM. 

The Pole-Changer.— The pole-changer, shown in Fig. 
no, is constructed with a lever operated by an electro- 
magnet in opposition to a retractile spring, by means of 
a small local battery and connected key. The end / of 




Fig. no. 



this lever projects, without contact, through an opening 
in a metal disk D, and has a free vertical movement be- 
tween two curved, metal springs s and s' , connected 
With opposite poles of the main battery; s being, for 



THE ELECTRIC TELEGRAPH. 335 

convenience, supposed to be connected with the positive 
pole, and s' with the negative. These springs are 
attached to the disk by insulated connections, and in 
prpximity with them are two metal blocks b and b' 
attached to the disk by uninsulated connections, and hav- 
ing adjustable stops with which each spring can make 
contact alternately. The disk is connected with the 
line wire, giving these blocks a line connection, and the 
lever, which is insulated from it, is connected with the 
earth wire. 

When the key is depressed and the lever attracted, it 
makes contact above with spring s\ lifting it and 
breaking its contact with block b' and also its own 
contact with j-, which therefore springs up into contact 
with block b. Hence the positive pole of the battery 
being now connected with the line through s, current 
flows from it to the line through s and b^ and thence 
to the earth through the apparatus at the distant sta- 
tion; and the negative pole being connected with the 
earth through / and s\ current flows to it from the 
earth through / and s' , and throi.gh the battery to the 
positive pole, completing the circLit. 

But when the key is opened, the attraction ceases, 
and the lever being pulled down by the retractile spring, 
makes contact below with j-, depressing it and breaking 
its contact with b and also its own contact with s\ 
which therefore springs down into contact with b' 
Hence, the negative pole of the battery being now con- 
nected with the line, the direction of the current is 
reversed, and it now flows from the positive pole 
through s and / to the earth, and from the earth at the 
distant station through the apparatus to the line, and 
from the line through b' and s' to the negative pole> 
and through the battery to the positive pole, complet- 
ing the circuit. 



336 DYNAMIC ELECTRICITY AND MAGNETISM. 

Hence, the polar connections being reversed with 
each opposite manipulation of the key, the direction of 
the line and earth currents is correspondingly reversed, 




while the direction of the current through the battery 
remains, of course, unaltered. 

The Polarized Relay.— The polarized relay, shown m 
Fig. Ill, is constructed with a curved steel magnet, 



THE ELECTRIC TELEGRAPH. H^ 

whose north pole, iV, we may, for convenience, suppose 
to be at the upper end, and its south pole, 6", at the 
lower end, as marked. On the south pole is mounted 
an electromagnet, attached by its soft-iron yoke F, Y, 
having oppositely wound coils, M and J/", between whose 
cores the soft-iron armature a, hinged to the north pole, 
has a free horizontal movement, limited by the stops 
attached to c and c, with which its projecting brass 
tongue b makes contact alternately. 

This armature derives north polarity from its attach- 
ment to the north pole of the permanent magnet, while 
the cores of the electromagnet derive south polarity 
from their connection through the yoke with the south 
pole. Hence, when there is no current in the electromag- 
net, its poles exert equal and opposite attraction on the 
armature; and the same is true when currents of equal 
strength flow through both coils, neutralizing each 
each other's magnetic effect. But when current flows 
through only one coil, or is stronger in one than in the 
other, the permanent magnetism of the cores is over- 
come, and each acquires polarity in accordance with the 
direction of the current. Hence the armature, having 
north polarity, is attracted by the south pole and 
repelled by the north, and therefore vibrates between 
the poles in response to the changes in the direction of 
the current, produced by the pole-changer through the 
manipulations of the key; alternately closing and open- 
ing the local circuit of the connected sounder. 

The various connections are made through three pairs 
of binding-posts on the left; and the positions of. the 
electromagnet, armature, and contacts, and the range 
of the armature's movement, are adjustable by screws, 
as shown. 

It is practically impossible to prevent the armature 
fiom being attracted against one of the stops when 



338 DYNAMIC ELECTRICITY AND MAGNETISM. 

there is no current in the magnet coils, notwithstanding 
the equal and opposite attraction; but such contact is 
then of no consequence, and, when the instrument is in 
operation, the armature is always so attracted, normally, 
by the electromagnetism. 

Operation of the Polar Duplex. — The connections and 
operation of the polar duplex, at opposite stations, are 
shown in Fig. 112. The line circuit, and the earth, or 
"artificial," circuit have equal resistances, as in the 
S-tearns duplex. 

Let battery B have its positive pole to the line and 
its negative to the earth, and the corresponding polar 
connections in battery B' be reversed. The positive 
current from battery B then divides at 6'' into currents 
of equal strength, traversing the oppositely wound coils 
a and b of relay R without magnetic effect; but the cur- 
rent through a is doubled by the negative current of 
battery B\ which flows in the same direction, producing 
a south pole in the core of a and a north pole in that of 
^, and hence armature M^ having north polarity, is 
attracted to the pole of a and repelled from the pole of 
b. The negative current of B' flows in through coils a' 
and b' of relay R' equally, producing no magnetic effect, 
but the current through coil a' is doubled by the inflow- 
ing positive current from B^ producing a south pole in 
the core of a' and a north pole in that of b' \ hence 
armature L' is attracted to pole of a' and repelled from 
pole of b' . m 

Supposing the above conditions to exist when mes-J 
sages are about to be sent in opposite directions simul- 
taneously; let the connections through pole-changer A' 
be reversed by depression of the key, putting the positive 
pole of B' to the line; the two batteries being now in 
opposition positivel)?", there is no current in the line, but 
there is still a current to the negative pole of each bat- 



THE ELECTRIC TELEGRAPH. 339 




I 




34<^ DYNAMIC ELECTRICITY AND MAGNETISM. 

tery from the earth. This current flows to the negative 
pole of B from right to left through H^ coil ^, and pole- 
changer A\ and hence reverses the polarity of relay R, 
attracting M from pole of a to pole of b\ while that to 
the negative pole of B' flows as before, from left to 
right, and hence produces no change of polarity in R\ 
whose armature M' therefore remains attracted to pole 
of a' as before. 

Now let the polar connections of B' be again reversed, 
and the former connections being restored, the current 
through b of relay R is reversed, and M attracted from 
pole of b to pole of a^ while M' is still attracted to pole 
of a^ as before. Hence relay R responds to the changes 
of polarity produced by pole-changer A\ while relay -^' 
'is unaffected by them. 

If, under either of the above conditions, the polar 
connections of B be reversed, corresponding effects are 
produced in relay R' ^ while relay ^ remains unaffected. 

Sounders, connected with each relay, are operated by 
local batteries in the usual manner. There is also a 
small rheostat connected with each condenser, as shown, 
to regulate its action by retarding its discharge, other- 
wise liable to be premature on long lines. 

Quadruplex Telegraphy.— Dr. J. B. Stark of Vienna, in 
1855, invented the first experimental method of dipleXy 
or double, transmission, in the same direction, on a 
single wire. 

Dr. Bosscha of Leyden also invented a diplex method, 
at the same time; and Kramer, during the same year, 
made an improvement in Stark's method, and Maron of 
Berlin, in 1863, improved Bosscha's method. But none 
of these methods came into practical use, though bene- 
ficial in opening the way for future inventors, and no 
further progress in diplex transmission was made till 



THE ELECTRIC TELEGRAPH. ■ 34 1 

after the practical adoption of the Stearns duplex sys- 
tem in 1872. 

Dr. Stark, in describing his system, in October 1855, 
was the first to show that diplex transmission could be 
combined with duplex, so that four messages could be 
transmitted simultaneously in opposite directions on 
the same wire; but he did not think such a system could 
become practical. Dr. Bosscha also showed the possi- 
bility of simultaneous quadruple transmission, in describ- 
ing his method. 

In 1873, Oliver Heaviside, an English electrician, 
showed that not only was simultaneous quadruple 
transmission practical, but also simultaneous multiple 
transmission, to an indefinite extent; so that, theoreti- 
cally, any required number of messages could be trans- 
mitted simultaneously in opposite directions. Multiple 
transmission, to this extent, has not yet been practically 
realized; but it has been experimentally shown to be 
possible, to the extent of eight or even sixteen messages, 
while quadruple transmission has long since become a 
practical success. 

In 1874 Edison, in connection with Prescott, invented 
the first quadruplex telegraph. It was constructed on 
the principle of the Wheatstone bridge, and was oper- 
ated between New York and Boston by the Western 
Union Telegraph Company. In 1875 and 1876 Gerritt 
Smith made important improvements in this system, and 
the quadruplex, as now constructed, is the result of the 
combined labors of Edison, Smith, and Prescott, in im- 
proving, combining, and applying apparatus previously 
invented by others. 

Construction and Operation of the Quadruplex. — The Edi- 
son quadruplex system, as now operated by the Western 
Union Telegraph Company, is a combination of the 
Stearns duplex and polar duplex, in such a manner that 



^42. DYNAMIC ELECTRICITY AND MAGNETISM. 



the two methods can be operated simultaneously, on 
the same line, without interference. Fig. 113 shows the 
construction and connections at two terminal stations, 



"^"rMi 








A and B. The Stearns, or neutral^ relays, as they are 
called in distinction from the polar, are shown at iV^ and 
iV\ and their connected transmitters at T and T^\ the 



THE ELECTRIC TELEGRAPH. 343 

polar relays, at R and F}^ and their connected pole- 
changers at P and P^. The artificial lines are con- 
structed, as in the polar duplex, with small rheostats 
to regulate the action of the condensers. The main 
local batteries are shown at X and X\ supposed, for 
convenience, to consist of 150 cells each, and to be 
tapped at z and z\ so as to divide each into two bat- 
teries of 50 and 100 cells, so that the entire battery may 
be employed by a through connection, or only the 
smaller part, by a connection through the tap. The 
rheostats h and ^^ each having a resistance equal to 
that of the larger portion of the battery, are placed in 
the tap circuits, so as to maintain a constancy of resist- 
ance, equal to that of the entire battery, when only the 
smaller portion is in circuit. 

The neutral relays are constructed with short cores, 
wound with coarse wire, and have strong retractile 
springs; the object being to reduce their sensitiveness: 
while the polar relays, whose permanent magnetism 
tends to make them sensitive, are constructed with long 
cores, wound with fine wire, to increase their sensitive- 
ness; the resistance of the polar relay coil being about 
double that of the neutral relay coil. 

This difference in sensitiveness tends to make the two 
relays independent of each other, so that the polar relay 
can be operated by a light current which does not affect 
the neutral relay; while the strong current required for 
the operation of the latter does not interfere with the 
simultaneous operation of the former. 

The battery current passes through both the trans- 
mitter and the pole-changer, furnishing the constancy 
of current required by the polar relay, which is oper- 
ated, as has been shown, by the change of polarity pro- 
duced by the pole-changer, and, from its sensitiveness, 
responds with equal facility to the weaker current or 



344 DYNAMIC ELECTRICITY AND MAGNETISM. 

the stronger; while the neutral relay, which requires 
intermissions in the current, responds only to the 
stronger current. And the battery connections of the 
transmitters, being arranged as shown, the weaker and 
stronger currents are alternately admitted to the line 
by the respective up and dow,n movements of the trans- 
mitter levers, which alternately close the circuits through 
the smaller and larger batteries respectively. 

The earth connections of the pole-changer levers are 
shown at G and G\ and those of the artificial circuits 
at E and E' . The keys and sounders, with their local 
circuits, omitted in the diagram, are arranged in the 
usual manner; as are also the construction and winding 
of the relays, to which an ideal construction and wind- 
ing are given in the diagram. 

The keys at A being closed and those at B open, the 
current from the positive pole of battery X flows through 
the right hand post and tongue of transmitter 7" and 
lower tongue of pole-changer P^ through both coils of 
neutral relay N and polar relay R with currents ol 
equal strength; the right-hand current going through 
the artificial line to the earth at E^ and the left hand 
current through the line to^; through coils a' of relays 
R* and N\ the upper tongue of pole-changer Z'', the 
tongue and center post of transmitter T^ to the negative 
pole z^ of the smaller battery at X\ through this bat- 
tery to the positive pole, thence through the lower 
tongue and lever of P^ to the earth at G'; from the 
earth at G^ through the lever and upper tongue of pole- 
changer /*, to the negative pole of battery X^ and 
through the battery to the positive pole, completing 
the circuit. 

There is now a 150 cell positive current, from battery 
X^ traversing both coils of relays iVand R equally, and 
hence producing no magnetic effect on them. But this 



THE ELECTRIC TELEGRAPH. 345 

current is increased in the left hand coils, a and ^, by 
the 50 cell negative current of the small battery at X' \ 
and this increase produces sufficient magnetic effect to 
attract the armature of relay R to the left hand stop, as 
shown, but not sufficient to attract that of N against 
the force of its spring. But this 200 cell current trav- 
erses the right hand coils, a' and a\ of both relays at B^ 
magnetizing their cores, so that the armature of relay 
N^ is attracted to its left hand stop, as well as that of 
relay K" to its right hand stop, as shown, producing a 
down click in the connected sounder of each relay. 

Now let the key of pole-changer P^ be closed, revers- 
ing the polarity of the smaller battery at X\ so that 
its positive pole is to the line, the current coming from 
the earth at G' . Its 50 cell positive current now flows 
through both coils of relays N^ and R^ equally, produc- 
ing no magnetic effect; the current through the artifi- 
cial branch going to the earth at E' ^ while that through 
the line branch changes the inflowing 50 cell negative 
current to an outflowing 50 cell positive current; so 
that now the inflowing current through a' and a^ of 
both relays is only that of 100 cells; but, on account of 
the opposite winding in each branch, this 100 cell in- 
flowing current, on the right, and the 50 cell outflowing 
current, on the left, both magnetize each core in the 
same sense, so that both armatures are held in their 
former positions by the magnetism produced by a 150 
cell current. 

But the current through a, of relay j^, being also thus 
reduced from that of 200 cells to that of 100, while the 
current through b still remains that of 150 cells, as 
before, armature W x's, attracted by the magnetism of a 
50 cell current from the left to the right stop, producing 
a down click in the connected sounder, while the same 



34^ DYNAMIC ELECTRICITY AND MAGNETISM, 

preponderance of current does not affect the armature 
of relay N. 

Now let the key of transmitter T^ be closed, and the 
connection to Z^ through the transmitter tongue, lever, 
arid center post being opened at ^, and that through its 
tongue and left hand post, to the negative pole of the 
full battery at X' closed; the 150 cell positive current 
of X' being now to the line, neutralizes the 100 cell 
positive line current from battery X, but adds a positive 
current of 100 cells to the 50 cell current through the 
artificial line at B\ the armatures of both relays at B 
are therefore still held attracted as before. But the 100 
cell line current through coils a and a at station A be- 
ing neutralized, while the 150 cell current through b 
and b an<i the artificial line still remains, the magnetism 
in relay W becomes strong enough to attract the arma- 
ture, producing a down click in the connected sounder, 
while the armature of relay R is still held on the right 
hand stop as before. 

Now let the connections of battery X' be reversed by 
opening the key of pole-changer P^^ putting the nega- 
tive pole to the line. The negative current of X', being 
now added to the positive current of X, produces a line 
current of 300 cells, which flows in through coils a* and 
a\ holding the armatures of both relays at B attracted 
as before, being in the opposite direction to the former 
outgoing current through b' and b\ which has ceased, 
and hence magnetizing the cores in the same sense. It 
also flows out through coils a and a^ at station A^ giving 
them a preponderance of 150 cells of current, over the 
current through b and b. The armature of relay N is 
therefore held attracted as before, while that of relay R 
is attracted from the right to the left stop, producing 
an up click in the connected sounder. 

Now let the key of transmitter T^ be opened, and the 



THE ELECTRIC TELEGRAPH. 347 

result is a change of the negative connection of battery 
X' to that of the smaller battery at Z^, restoring the 
conditions first considered, with both keys open at B 
and both closed at A\ releasing the armature of relay 
N^ which is pulled back by the spring, producing an up 
click in the connected sounder, all the other armatures 
remaining undisturbed. 

All the conditions which can occur with both keys 
closed at one station, and one or both keys either opened 
or closed at the other station, have now been considered; 
the results being, evidently, practically the same with 
the operations at the respective stations reversed: and 
it has been shown that, in each case, both relays, at 
each station, can be operated without mutual inter- 
ference; each responding to the manipulations of the 
key connected with the corresponding relay at the 
distant station, while the relays at the home station are 
unaffected by the manipulations of the home keys. 

There are minor points of practical importance, in the 
adjustment, which require attention, to prevent false 
signals, liable to occur during the momentary change 
of battery connections, and which, even with the best 
regulation, cannot be wholly prevented, but are not of 
sufficient importance to prevent the practical operation 
of the system. 

Repeating by the duadruplex. — The quadruplex system 
is also employed for repeating in a similar manner to 
that of the repeaters already described; messages being 
repeated either from one relay to another of the same 
kind, or from one side of the system into the other, as 
from neutral relay to polar, or the reverse; the latter 
being the usual method. 

Substitution of the Dynamo for the Battery. —The dyna- 
mo was first substituted for the battery in telegraphing 
by Stephen D. Field, and put in operation by the 



34^ DYNAMIC ELECTRICITY AND MAGNETISM. 

Western Union Telegraph Company at New York in 
1880, the Siemens-Halske dynamo being employed. 
But the result not being entirely satisfactory, the bat- 
tery was reinstated in 1887. Meantime great improve- 
ment had been made in dynamo construction, and in 
1890 the dynamo was again substituted for the battery, 
not only at New York, but at all the principal stations 
of the Western Union. The improved Edison dynamo 
was adopted; the no volt machines, grouped in series 
of five each, and operated at a working potential of 70 
volts to each machine, being employed for the line cir- 
cuits, and the 5 to 7 volt machines for the local circuits; 
a single dynamo being sufficient for the operation of 
several of the latter circuits, arranged in parallel. 
. Some changes are required to adapt telegraphic 
transmission to the dynamo current, especially in the 
use of the duplex and quaduplex systems. The re- 
versals of polarity are made between two series of 
dynamos, one series furnishing the positive current and 
the other the negative. And the increase and decrease 
of current is produced by causing the currents to 
traverse routes of different resistance. 

By the use of the dynamo a full supply of current 
can be obtained at far more economical rates than by 
the battery, and also greater constancy of current; the 
loss due to battery exhaustion being obviated. 

The Wheatstone System of Automatic Rapid Transmis- 
sion. — As telegraphic transmission by manual manipula- 
tion of the key does not exceed 25 to 50 words per 
minute, it becomes important in telegraph offices doing 
a large business to have some more rapid method. 
This is furnished by the system of automatic rapid 
transmission invented by Wheatstone, and employed in 
Great Britain for the Postal Telegraph, and in the 
United States by the Western Union at all its principal 



THE ELECTRIC TELEGRAPH. 349 

offices. Its relations to manual transmission are similar 
to those of printing to writing. It consists in the prep- 
aration of the message for transmission by recording 
it with perforations made in a strip of tough paper, and 
the subsequent automatic operation of a transmitter by 
this perforated strip, by which the message is transcribed 
in Morse characters on a similar strip by an inking 
register at the distant station. 

Its instruments are a perforator and a transmitter, 
both of special construction, and an inking register of 
the ordinary type. The perforation of the paper is a 
purely mechanical operation. Three parallel rows of 
holes are perforated by punches which cut out the bits 
of paper; the central holes, which are smaller and less 
perfect than the others, being made in a continuous 
series, equally spaced, by the teeth of a star-wheel by 
which the strip is simultaneously drawn through, while 
the message is punched in the two side rows. The 
punches are operated by three keys pressed down by 
two little mallets in the hands of the operator against 
the force of spiral springs which bring them up again. 
The depression of the left-hand key punches a hole on 
each side of the central hole, preparing the paper for 
the transmission of a dot. The depression of the cen- 
tral key carries the paper forward one wheel-tooth 
space, making only the central hole, preparing the paper 
for the transmission of a space, the length of which can 
be doubled by two consecutive depressions of this key. 
The depression of the right-hand key carries the paper 
forward two wheel-tooth spaces, making a hole on the 
left when the first wheel-tooth space is passed, and on 
the right when the second is passed, each opposite a 
central hole, preparing the paper for the transmission 
of a dash. The appearance of a strip prepared in this 
manner for the transmission of the word ^^ operator'' is 



350 DYNAMIC ELECTRICITY AND MAGNETISM. 

shown in Fig. 114, a care ul examination of which will 
verify the above description. 

The paper, when thus prepared, is placed on the 
transmitter, and carried forward at a regular rate of 
speed by a star wheel whose teeth fit into the central 
holes, and which is operated by a spring or weight. 
Each side row of holes passes over the points of two 
vertical rods, connected with light apparatus contained 
in a box underneath, by which a pole-changer is oper- 
ated. These points are pressed against the paper by 
spiral springs connected with the rods by bent levers, 
which operate the pole-changer; their upward move- 
ments being limited by two stops attached to an insu- 




Op & T a t r 

Fig. 114. 

lating walking beam, to which a regular, alternate 
motion is imparted by the force which operates the star 
wheel. As each point meets a hole in the moving 
paper it passes up till its connected lever meets a stop 
on the walking beam below; and each alternate move- 
ment of this beam, made simultaneously with the ad- 
vance of the paper one tooth space, pushes down the 
point which is up, and permits the other point to ascend 
till it is either stopped by the paper or passes up through 
a hole. 

The ascent of the left-hand point through the per- 
forated paper connects the positive pole of the battery 
with the line, bringing the pen of the register, at the 
distant station, into contact with the receiving paper; 



THE ELECTRIC TELEGRAPH. 351 

and the ascent of the right-hand point through the per- 
forated paper reverses the polarity, withdrawing the 
pen from the receiving paper. And as the left point is 
adjusted one tooth space in advance of the right, if the 
two holes intended to transmit a dot pass over the 
points, the left point will ascend through its hole first, 
and be immediately depressed and the right point as- 
cend through its hole; and hence the registering pen 
will touch the paper and be at once withdrawn, making 
the dot. 

If now the paper move forward one tooth space, and 
the points meet no hole on either side, the negative 
pole being still to the line, and hence the registering 
pen still withdrawn, a space occurs on the receiving 
strip, the length of which depends on the distance the 
perforated strip moves before the left-hand point meets 
a hole. When this point meets a hole, the polarity 
being again reversed and a positive pole put to the line, 
the registering pen again touches the paper and is kept 
in contact till the right hand point meets a hole. If 
two tooth spaces are passed before this occurs, a dash 
is registered on the receiving strip, as shown at a or /, 
Fig. 114; but if only one is passed, a dot is registered, 
as before, as shown at e. 

Hence it appears that the office of this perforated 
paper is simply to reverse the polarity, and that when 
the positive pole is put to the line, either a dot or a dash 
is registered according to the time elapsing before re- 
versal; and when the negative pole is put to the line, 
either a short or a long space is registered according to 
the time elapsing before reversal. In this way dots, 
dashes, and long and short spaces can be registered 
automatically as rapidly as the instruments can be 
made to operate; the usual range of speed being from 



352 DYNAMIC ELECTRICITY AND MAGNETISM. 

125 to 250 words per minute, which is about five times 
that of manual transmission. 

Hence one wire, by this method, can do the work of 
five wires by the ordinary method, and thus a large 
volume of telegraphic business be rapidly disposed of. 
This is especially advantageous in case of an accidental 
break in the connections, such as often occurs, since 
messages can still be received and prepared for trans- 
mission, and the accumulation rapidly despatched when 
the break is repaired. The simultaneous transmission 
of duplicate press despatches to numerous points from 
a central station can also, in this way, be greatly 
facilitated. 

Submarine Telegraphs. — Submarine telegraph lines are _ 
constructed with cables like that shown in Fig. 115. ■ 




Fig. 115. 

Seven or more No. 16 copper wires, thoroughly insu- 
lated, are inclosed in jute and protected by an exterior 
armor of iron wires wrapped with hemp; the interior 
being made water-proof. Deep sea cables, such as are 
used for the Atlantic lines, are made much lighter than 
those designed for shallow water, or shore ends, where 
the cables are more exposed to injury. 

There are two important points of difference between 
the operation of long ocean lines, like those across the 
Atlantic, and ordinary land lines. One is, that the use of 
powerful electric currents, such as are required to oper- 



THE ELECTRIC TELEGRAPH. ^353 

ate the ordinary Morse instruments on long land lines, 
are liable to damage the insulating material of long sub- 
marine lines, and produce faults which soon render such 
lines worthless. The other is, that the peculiar condi- 
tions of great length, submergence in water, and an 
incasing armor of conducting material, insulated from 
the interior conductors, produces an excessive static 
charge, similar to that of the Leyden jar, which seri- 
ously impedes electric transmission. 

Hence the first requisite is very sensitive apparatus, 
capable of responding to a low accompanying current, 
just sufficient to operate it, but not to injure the insula- 
tion ; such a current also reducing the static charge to 
its minimum. The second requisite is a condenser by 
which an induced, operating current is sent to the line 
and also a subsequent, reactive, opposing current, which 
neutralizes the return current of the static charge in the 
manner already described. 

Thomson's reflecting galvanometer and accompany- 
ing scale, described in Chapter VI, page 130, furnishes 
the sensitive apparatus required, and is employed as 
the receiving instrument; the movements of the spot 
of light, on the scale, to the right or left of the zero 
mark, being made to indicate dots in one direction and 
dashes in the opposite direction, the pauses at zero indi- 
cating spaces. 

"Wiom^on ^ siphon recorder^ also a very sensitive instru- 
ment, is employed for the same purpose. It consists of 
a capillary glass siphon, lightly poised, to which an 
oscillating, horizontal movement is given by apparatus 
operated by the line current through a local circuit, by 
which its point is made to vibrate, without contact, 
across a moving strip of paper, making a continuous, 
irregular line of dots with ink ejected in fine drops, 
drawn from a vessel in which the opposite end of th" 



354 DYNAMIC ELECTRICITY AND MAGNETISM. 



siphon is immersed; the ink and paper being oppositely 
electrified, and the various contortions of the line made 
to indicate the Morse characters. 

Fig. ii6 represents a cable with the transmitting 
G 




CABLC 




Fig. 1 1 6. 

apparatus in connection at the terminal station on the 
right, and the receiving apparatus at the terminal sta- 
tion on the left, each kind being, of course, duplicated 
at each station, and the connections for transmitting or 
receiving changed by switches as required. The trans- 
mitting apparatus consists of the key JT, permanently 
connected, by its rear contact, with the battery By and 
arranged for connection by its front contact, when de- 
pressed, with the earth at E^ with which the battery is 
also connected by its opposite pole. The receiving ap- 
paratus consists of the galvanometer Gy connected with 
the earth at E' y through the condenser C, on one side, 
and through the rheostat Ry on the other. The con- 
denser contains 40,000 or more square feet of tinfoil, 
and hence is capable of accumulating a very large 
charge, and the rheostat has a very high resistance. 

The connection of the negative pole of the battery 
with the line through Ky and of the positive pole with 
the earth, sends a positive current to the earth at Ey and, 
as a result of the negative potential thereby produced in 



THE ELECTRIC TELEGRAPH. 355 

the line, a positive current flows from the earth at E' 
into the conductor C, repelling an equal amount of 
electricity from the opposite side of the condenser to 
the line and negative pole of the battery, completing 
the circuit. The line and line side of the condenser 
thus become negatively charged, but being at the same 
potential on both sides of the galvanometer G^ this 
charge does not affect the needle after the first deflec- 
tion produced by the charging ; the needle therefore 
remains at zero. Now let the key at K be depressed, 
closing the front contact, and a positive current flows 
to the line, and into the line side of the condenser, from 
the earth at E, producing a deflection of the needle ; but 
the condenser and line becoming charged positively, to 
the same potential, on both sides of the galvanometer, 
the needle returns immediately to zero. Now let the 
front contact at K be opened, and the former condition 
being restored, and the current reversed, a deflection 
in the opposite direction occurs, after which the needle 
returns to zero as before. 

The advantage of a constant charge of the line in this 
manner becomes apparent when we consider that the 
complete charging or discharging of a submarine cable, 
two or three thousand miles in length, occupies several 
minutes, the time varying in proportion to the square 
of the length. The currents transmitted by the manipu- 
lations of the key rise and fall in waves ; about y^- of a 
second elapsing before any perceptible effect is produced 
at the opposite station by the closing or opening of a 
battery connection, and about 3 seconds being required 
for the wave to attain its full strength, and 3 more for it 
to decline. Hence, if the full phase of a wave inter- 
vened between signals, transmission would be exceed- 
ingly slow. But, by keeping the line charged, and em- 
ploying the condenser, the current can be quickly re- 



35^ DYNAMIC ELECTRICITY AND MAGNETISM. 

versed after each signal, without waiting for the wave 
to attain its full strength. And as the deflections indi- 
cating dots and dashes respectively are in opposite 
directions, their amplitude is of no consequence. Hence 
if, for instance, a succession of dots is required, as for 
the letter ZT, the first deflection has large amplitude, 
but is checked by a momentary reversal of current, 
and then further increased by another reversal; and 
thus, by four impulses in rapid succession, each less 
prominent than the preceding one, but all in the same 
direction, the four dots required are indicated. In like 
manner dashes are indicated by similar opposite deflec- 
tions. A speed of 15 to 20 words per minute can thus 
be attained. 

The constant charge of the cable and condenser in 
this manner reduces the interference of earth currents, 
often serious on lines of such length, to its minimum, 
so as to be of no practical importance ; the charge being 
sufficient to counteract the effect of such currents, under 
ordinary conditions, and leave a working surplus. 

The earth connection at O allows a small percentage 
of current to pass through the rheostat, by which the 
potential on opposite sides of the galvanometer is equal- 
ized when the needle is at zero, without interference 
from the cable charge ; this potential being nearly the 
same as that of the line, on account of the high resist^ 
ance of R. 

Locating Faults. — When a fault or break occurs in a 
submarine line, it can be located approximately by the 
Wheatstone bridge test, invented by De Sauty. One 
arm of the bridge is connected with the cable, and the 
opposite arm with a condenser of known resistance ; 
and a current being sent through the instrument, flows 
into the condenser, through one side, and into the cable 
through the other, and to the earth at the fault ; and 



THE ELECTRIC TELEGRAPH. 357 

by this means the resistance from the shore to the fault 
can be ascertained. And the cable resistance per mile, 
in ohms, being known, the distance to the fault is easily 
ascertained. 

As the resistance of the fault itself often varies con- 
siderably, it is important to test from each end and com- 
pare the results. And by subtracting the added results 
from the known resistance of the cable, the loss of re- 
sistance due to the fault can be accurately ascertained ; 
and the proportionate amount of the remaining resist- 
ance from each end should give the resistance to the 
fault, from which the distance in miles can be ascer- 
tained as before. 

When a cable contains separately insulated conduc- 
tors, and a fault occurs in one of them, the loop method, 
which is considered one of the most accurate, can be 
adopted. Let the faulty wire be connected with a per- 
fect wire at one end of the cable, and the two opposite 
terminals connected with the bridge at the other end ; 
a current being transmitted, goes to the earth at the 
fault, passing round the loop from one side of the 
bridge, and direct to the fault from the other side. 
The known resistance of the cable being subtracted 
from the ascertained resistance of the loop, the re- 
mainder is the resistance to the fault from the end 
where the two wires are joined. The resistance from 
each end to the fault being thus ascertained, the dis- 
tance can be calculated. 

The Dial Telegraph. — In the dial telegraph, different 
forms of which have been invented by Wheatstone, 
Breguet, and Siemens, messages are indicated by ordi- 
nary letters displayed on a dial. In Breguet's receiving 
instrument the letters of the alphabet are arranged on 
the dial in a circle, around which a pointer rotates, 
stopping at the required letter. This pointer is oper- 



358 DYNAMIC ELECTRICITY AND MAGNETISM, 

ated by an electromagnet by means of clock-work in 
response to the closing or opening of the circuit by 
the transmitter; each make or break moving it one 
letter, always in the same direction ; hence by a number 
of such changes in rapid succession it is brought to the 
required letter. 

The transmitter has a similar dial around which a 
lever is moved, by which a toothed wheel is rotated, 
which either closes or opens the circuit alternately at 
each letter, by a connected lever, till the required letter 
is reached. 

Such telegraphs are well adapted to the requirements 
of private lines, where the services of skilled operators 
are not available, and have been so employed in Europe ; 
but, in the United States, printing telegraphs have been 
preferred for such lines. 

Printing Telegraphs. — Telegraphs by which the mes- 
sage is printed in ordinary type have been invented by 
House, Hughes, Phelps, and Pope and Edison. A de- 
tailed description of these various instruments can be 
found in " Prescott's Electricity and the Telegraph" 
and similar books, to which the reader is referred. 
Their general principles of construction are similar 
to those of the dial telegraphs, and may be illustrated 
by supposing the lettered keys of a type-writer con- 
nected with telegraphic apparatus in such a manner 
that the depression of a key transmits a current by 
which a letter, corresponding to the one marked on the 
key, is printed on a strip of paper in the receiving in- 
strument at the distant station, and the paper moved 
as required by the same apparatus. In fact, the print- 
ing telegraph is such a telegraphic type-writer. 



THE TELEPHONE. 359 



CHAPTER XIIL 

THE TELEPHONE. . j^ 

Early History. — The reproduction of music and speech 
by electric transmission was first accomplished by Phil- 
ipp Reis of Friedrichsdorf in 1861, though the transmisr 
sion of sound and speech by the ordinary vibrations of 
a tightly stretched wire, as in the mechanical telephone, 
had been known for 200 years previous. The produc- 
tion of sound by electromagnetic vibrations was first 
observed by Page in 1837, in connection with the alter- 
nations of magnetism induced in an iron bar by an in- 
termitting electric current in proximity ; which-j when 
occurring rapidly and rhythmically, gave rise to a musi- 
cal tone. In 1854 Bourseul described a method which 
he had tried, by which speech could be electrically 
transmitted, which is practically the same as that now 
employed ; and predicted its ultimate success when 
sufficiently developed. During the next twenty years 
the only progress made consisted in improvements of 
the musical telephone by various inventors. 

About 1874 Elisha Gray began a series of experiments 
on the musical telephone, in Chicago, which led to the 
invention of a method by which it could be employed for 
the transmission of speech, for which, in 1876, lie filed 
a caveat in the United States Patent Office. Meantime 
Alexander G. Bell had been making similar experiments, 
in Boston, expressly for the transmission of speech, 
and he also invented a method for which he applied 
for a patent on the same day that Gray filed his caveat. 



360 DYNAMIC ELECTRICITY AND MAGNETISM. 

A patent was granted to Bell March 7, 1876, his tele- 
phone was exhibited at the Centennial exposition at 
Philadelphia the same year, and its practical application 
to commercial use soon followed. 

Claims for priority of invention were made by Gray, 
also by Daniel Drawbaugh of Pennsylvania, who claimed 
to have invented a similar telephone in 1872, and by Dr. 
Cushman, whose claims extended back to 1851. The 
litigation between Bell and Gray ended in a compromise, 
and that with the other parties was decided in Bell's fa- 
vor. The transmission of speech by Reis's telephone was 
so imperfect as not to be considered entitled to priority 
as against the more perfect method invented by Bell. 

Principles of the Telephone. — Sound, in the telegraph, 
is the arbitrary symbol of speech, while that in the tele- 
phone is its reproduction, the result of undulations of the 
air produced by the speaker at one end of the line, and 
reproduced by the transmitted currents at the other end. 
Tones, whether of music or articulate speech, are caused 
by the occurrence of such undulations in rhythmical 
order, their pitch depending on the number of undula- 
tions per unit of time, and their volume on the ampli- 
tude of those undulations. The property known as 
timbre^ which distinguishes tones of the same pitch and 
volume from each other, depends on the manner in 
which the undulations are produced by different voices; 
a graphic representation showing undulations of differ- 
ent form. 

In the telephone these three properties, pitch, volume, 
and timbre, are accurately reproduced by the undula- 
tions, so that the characteristic quality of the voice, and 
the manner of utterance, can be distinctly recognized, as 
well as the words spoken. And these undulations are 
produced by variations of current strength, and not by 
intermissions of current as in the telegraph. 

It is the property of reproducing sonorous tones 



THE TELEPHONE. 



361 



which was first recognized, and caused the invention of 
the musical telephone to precede that of the speaking 
telephone, the properties of articulation and timbre 
being subsequently developed; and it is the development 
of these latter properties which distinguishes the Bell 
telephone from the musical telephones of Reis and 
Gray. As the Bell telephone embodies the leading 
principles of the musical telephones of Reis and Gray, 
a detailed description of the latter is unnecessary. 

The Bell Telephone. — The Bell telephone is strictly a 
magneto-electric apparatus, generating its electric cur- 
rents by the movements of a magnetized armature in 
proximity to a conductor forming a closed circuit. The 
construction of its principal instrument, employed now 
as a receiver only, but formerly both as a transmitter and 
a receiver, is shown in Fig. 117. A round, hard-rubber 
case, 6J inches long and i^ inches in 
diameter, enlarged at one end as 
shown, incloses a round bar magnet, :j- 
of an inch in diameter, to whose north 
pole, N^ is attached a wooden bobbin, 
bb^ wound with fine, silk-covered, cop- 
per wire, whose terminals, gg, are at- 
tached to larger wires, c c, connected 
with the binding-posts h h. A very 
thin sheet-iron disk, PP^ 2^ inches in 
diameter, varnished to protect it from 
oxidation, covers the circular space 
within which the bobbin and magnet 
pole are placed, and is held in position, 
at its edges, by the ear piece W, which 
is screwed over it as shown. The 
center of this disk comes close to the 

magnet, the distance being adjusted 

t .1 J re ■ ^ u • Fig. 117. 

by the screw a; sufficient space being 

allowed for a slight vibration of the disk, without con- 




362 DYNAMIC ELECTRICITY AND MAGNETISM. 

tact. There is also a similar amount of space between 
the disk and the ear piece: and the vibrations produced 
by the variations of magnetic energy are limited by the 
elasticity of the disk; its center vibrating very slightly, 
while its edges are held fast. This disk is known as the 
diaphragm. 

In the bottom of the cavity of the ear piece, opposite 
the center of the diaphragm, is an opening, -^ of an inch 
in diameter, through which the undulations produced in 
the air by the vibrations are transmitted to the ear. 

If one terminal of the coil, as Z, be connected with an 
ordinary telegraph line, and the other, E^ with the earth, 
and corresponding connections be made with a similar 
instrument at the opposite end of the line, one instru- 
ment can be used as a transmitter and the other as a 
receiver. When a person speaks into the transmitter, 
the undulations of the air cause the diaphragm to 
vibrate ; each vibration varying the distance between 
the disk and the magnet, producing corresponding varia- 
tions of magnetism in the disk as an armature, which 
induce an alternating current of varying strength in the 
coil. This current, transmitted to the coil of the re- 
ceiver, at the opposite end of the line, reproduces like 
variations of magnetism in its diaphragm, and hence 
corresponding vibrations and undulations, which repro- 
duce the words spoken into the transmitter. 

The improved telephone, as above described, was 
patented by Bell Jan. 30, 1877; the chief claims of the 
patent being the substitution of the iron disk for the 
stretched animal membrane previously employed, andS 
the magneto-electric current for the battery current. 

Improved Transmitters. — It is evident that much of the 
energy of the transmitted voice is spent in overcoming 
the various interposed resistances, so that when heard 
in the receiver it is comparatively weak, and the feebler 



THE TELEPHONE. 363 

tones are riable to be indistinct. Hence various means 
have been devised to counteract the effects of this con- 
sumption of energy and render the delivery more dis- 
tinct. 

The Edison Transmitter. — It was observed by Du Mon- 
cell that increase of pressure reduces the contact resist- 
ance between conductors ; and that this effect is 
increased by reduction of hardness and increase of 
electric resistance in the conductors themselves: and 
hence that variations of current strength may be pro- 
duced in this way. It was also observed by Edison that 
carbon is peculiarly adapted to fulfill these conditions; 
and in accordance with these observed facts he con- 
structed, in 1878, the first transmitter in which carbon 
was employed. He also employed platinum, as pro- 
posed by Gray, using a disk of each material in contact, 
and producing a slight contact between the platinum 
disk and the vibrating diaphragm by an ivory button; 
the high resistance of each substance, the difference of 
their hardness, and the varying pressure produced at 
the numerous contacts by the vibrations of the dia- 
phragm, being intended to improve the conditions of 
transmission. 

Edison also employed an induction coil, as had pre- 
viously been done by Gray to increase the E. M. F. of 
the line current; connecting its primary coil with the 
circuit of a local battery, whose current traversed the 
transmitter, and its secondary with the line, by one 
terminal, and with the earth by the other, completing 
the circuit by corresponding connections of a Bell 
receiver with the line and earth at the opposite station. 
By this means the relative conditions of E. M. F., 
resistance and current strength, in the two circuits, 
could be varied, so that a large current of low E. M. F. 
and resistance, in the primary, could be converted into 



364 DYNAMIC ELECTRICITY AND MAGNETISM. 



a small current of high E. M. F., in the secondary, capa- 
ble of overcoming the line resistance, and producing 
sufficient amplitude of vibration in the 
diaphragm of the receiver to render the 
tones audible and distinct. 

The Blake Transmitter.— The transmit- 
ter invented by Blake, and novir employed 
by the Bell Telephone Company through- 
out the United States, is an improved 
form of the Edison transmitter. A ver- 
tical section of its principal parts is 
shown in Fig. 118, attached to the door 
of a little cabinet in which it is con- 
tained; Fig. 119 giving a rear view, with 
the door open, showing also the induc- 
tion coil and connections. 

Opposite the mouth-piece «, formed 





Fig. 119. 

by a funnel-shaped opening in the door, is fixed the 
bheet-iron diaphragm e, supported by a ,«ioft-rubber ring, 



THE TELEPHONE. 365 

?/?/, and the springs v v^\ v^ pressing on the ring, and 
V on the diaphragm itself, a piece of felt attached to 
the spring v intervening at the point of contact. This 
ring insulates the diaphragm from the conductors both 
of sound and electricity with which it is surrounded; 
and the felt on the spring prevents excessive vibration, 
reducing it to the normal quantity required for distinct 
transmission. An iron ring, rrrr, has two projections, 
b b, each about f of an inch in length, from the upper 
of which the iion bar, c, is suspended by the brass 
spring 7?i. A flat, metal springy, attached to the short 
upper arm of c, and inclosed in soft rubber to lessen its 
vibration, carries, at its lower end, a brass disk /, to which 
is attached a carbon disk k, both suspended opposite the 
center of the diaphragm e. A thin, flexible steel spring 
/, insulated from g by the block of vulcanized fiber/, is 
also attached to the upper arm of <:, its lower end hav- 
ing two platinum points, one of which makes contact 
with the center of the plate k, and the other with that of 
the diaphragm e. The tension of the springs/ and g^ 
and hence the pressure of the various contacts between 
the disks, is adjusted by the screw ;/, by means of its 
bearing against the lower, bent arm of c. 

Accessory Apparatus, — A Leclanche cell, in the lower 
compartment of the cabinet, supplies the current, the 
connections being through the primary circuit of the 
induction coil R to one of the door hinges, thence to 
the ring rrrr, upper projection b, upper arm of c, 
springs m and g^ plates/* and k, platinum point, spring 
/, through a lever in the upper compartment of the 
cabinet, and thence through the other door hinge to the 
opposite pole of the battery. The secondary circuit of 
the coil R is connected with the line and the earth by 
its opposite terminals. 

The lever referred to is used to open and close the 



366 DYNAMIC ELECTRICITY AND MAGNETISM. 

primary circuit. It terminates in an exterior forked 
hook in which the receiver is hung. The weight of the 
receiver depresses the lever in opposition to the force of 
a spring and opens the primary circuit, thus preventing 
exhaustion of the battery by the generation of current 
when not required. The receiver is connected with the 
line by a flexible conducting cord, and when taken off 
the hook to be applied to the ear, the lever, relieved of 
its weight, springs up and closes the battery circuit, 
sending a current through the transmitter and primary 
circuit of the induction coil, by which an induced cur- 
rent is generated in the secondary circuit and connected 
line. The message is then spoken before the mouth- 
piece of the transmitter. 

The Signaling Apparatus. — The signaling apparatus, 
invented by Gilliland, is also contained in the upper 
compartment of the cabinet and connected directly with 
the line. Its transmitter is a magneto-electric machine, 
operated by an exterior handle, by which a line current 
is generated which drops an annunciator at the central 
station. Its receiver is a call-bell^ constructed with two 
gongs, between which the clapper vibrates. The latter 
is operated by a double coil electromagnet, to the arma- 
ture of which it is attached. This armature is mounted 
a short distance in front of the poles, being pivoted at 
its center on one of the poles of a bar magnet, mounted 
between the coils. It has therefore the same polarity 
as the pole with which it is connected; hence when an 
alternating current, transmitted from the central station, 
passes through the coils one end of the armature is at- 
tracted and the other end repelled b}^ each pole alter- 
nately, as the polarity changes, producing a rapid, rock- 
ing motion which brings the clapper alternately into 
contact with each gong; 



THE TELEPHONE. 367 

The Exchange. — A telephone exchange is a central 
station in which intercommunication between all the 
subscribers' stations connected with the net-work of 
lines belonging to any particular local system is estab- 
lished. Each line radiates from this exchange, and all 
communication between subscribers must pass through 
it. Where the system is very extensive, embracing 
several thousand subscribers, there are also several dis- 
trict stations between which there are trunk lines by 
which connection between subscribers in different dis- 
tricts can be made. The terminals of all the lines are 
connected with a switch-board, which thus often be- 
comes a very extensive and elaborate apparatus. 

The Multiple Switch-Board. — A section of a multiple 
switch-board, such as is now employed by the Bell 
Telephone Company in the United States, is shown in 
Fig. 120. The supporting plates are composed of in- 
sulating material and pierced with holes through which 
connection is made with the various lines and connected 
apparatus attached to its back. Lady operators sit 
in front of it, each having a receiver to her ear and a 
transmitter suspended before her by which she can 
communicate with all the subscribers in her section. It 
is divided into compartments, A^ B, C, and £>. 

Compartment^ contains the annunciators which in- 
dicate the calls from subscribers, to each of whom one 
is assigned. They are constructed with hinged brass 
tablets, each having the subscriber's number on its inner 
face, and connected with an electromagnet by which a 
spring catch is operated, which keeps the tablet closed 
when not in use. When a subscriber transmits a cur- 
rent from his electromagnetic machine, it passes 
through the coil of this magnet, the armature is at- 
tracted, lifting the catch and releasing the tablet, which 



368 DYNAMIC ELECTRICITY AND MAGNETISM. 




Fig. 120. 



THE TELEPHONE. 369 

drops into a horizontal position and displays the sub- 
scriber's number. 

In compartment B are shown the flexible conducting 
cords for making connection with the subscribers' lines, 
kept stretched by suspended pulleys to prevent en- 
tanglement; each attached to a plug shown in the rear 
row, on the shelf, and passing through a connection 
controlled by a cam shown in the front row. Each of 
the holes, shown above the row of plugs, connects with 
a spring-jack in the circuit of each subscriber included 
in that section; while each of the holes in compartment 
Cconnects with a spring-jack in the circuit of each sub- 
scriber in the entire system; each wire in compartment 
C passing through a spring-jack in every section. 
Hence each operator can connect, in her own section, 
any of her subscribers with any subscriber belonging 
to her own or any other section, through compartment 
C; while in compartment B^ she can connect herself 
only with the subscribers included in her own section. 

In compartment Z> are the " clearing out" annunci- 
ators by which either of two subscribers, in communica- 
tion, can indicate the close of the conversation by a 
signal current which drops the connected annunciator 
in this compartment. On the shelf in this compartment 
are shown a row of plugs, each attached to the same 
cord as the corresponding plug in compartment B, so 
that any subscriber connected with a cord by the inser- 
tion of one of the lower plugs in the hole bearing his 
Jiunrber in compartment B^ can be connected with any 
other subscriber by insertion of the upper plug con- 
nected with that cord in the hole bearing the other 
subscriber's number in compartment C, as stated above. 
In front of these plugs is shown a row of buttons, each 
connected with a contact by which a circuit can be 
closed between any subscriber with whom connection is 



370 DYNAMIC ELECTRICITY AND MAGNETISM. 

made in compartment C and a magneto-electric ma- 
chine, and a call rung on the subscriber's bell; a number 
of these machines being kept in constant operation, at 
a large station, by a water motor or otherwise. 

When an annunciator drops, the operator places a 
plug, taken from the row in compartment B^ in a hole 
in one of the vertical rows immediately above it having 
the corresponding number, at the same time turning 
down the corresponding cam lever, so as to close the 
subscriber's circuit, and inquires what number is wanted; 
having ascertained this, she applies the terminal plug 
of her receiver to a special tube connected with the line 
of the subscriber called for, and if the contact produces 
noise in her receiver, it indicates that there is a current 
on the line, and she informs the subscriber that the per- 
son wanted is conversing with some one else. But as 
soon as she ascertains that the conversation has ceased, 
as indicated by silence when contact is made, she takes 
a plug from the row in compartment Z>, connected with 
the same cord as the plug inserted in the hole bearing 
the calling subscriber's number in compartment B^ and 
inserts it in the hole, in compartment C, bearing the 
number of the subscriber called for, which puts the two 
in communication; and having rung a call to the sub- 
scriber wanted, by pressure on the signal button, she 
opens the cam connection and closes the annunciator. 

All the spring-jacks through which any subscriber's 
line passes are in connection through a separate wire, 
and it is on a connection with this wire that the test 
above described is made. 

As each clearing-out annunciator is in circuit only 
when two subscribers are connected and can be operated 
by either, a comparatively small number is sufficient; 
but each subscriber being constantly in circuit with one 



THE TELEFHOiVE. 37 1 

of the other annunciators, there must be as many of 
these as there are subscribers. 

Hughes' Microphone. — In 1878 Hughes invented the 
instrument known as the microphone, by which feeble 
sounds can be transmitted, and reproduced, greatly 
amplified, in a telephone receiver. In his first experi- 
ments a wire nail, making loose contacts with two other 
nails, was employed, but when the superior quality of 
carbon for telephonic transmission was demonstrated 
by Edison, Hughes substituted a small carbon rod, 
pointed at both ends, and loosely mounted vertically 
between two carbon supports attached to a thin sound- 
ing board. The terminals of a battery circuit, in which 
the receiver is included, are attached to these supports, 
and the slightest sound made near this simple instru- 
ment, as the ticking of a watch or the walking of a fly, 
can be distinctly heard in the receiver, at a distance of 
several feet. 

The instrument is too sensitive to be used for ordinary 
transmission, but the advantage of loose contacts, as 
thus demonstrated, has been utilized to increase the 
sensitiveness of carbon transmitters, by the substitution 
of granulated carbon for carbon plates. 

Theory of Telephonic Transmission. — Opinion is divided 
in regard to the fundamental principles of telephonic 
transmission, and especially in regard to the functions 
of carbon as a transmitter. It is well known that heat- 
ing reduces the electric resistance of carbon, and hence 
it is maintained that the variation of heat generation 
in the carbon, produced by the variation of pressure 
due to the loose contacts, produces a corresponding 
variation of resistance and hence of current strength. 
While the heat thus generated must be almost infinites- 
imal in quantity, nevertheless its ratio to the molar and 
molecular vibrations, in an apparatus of such delicate 



l'J2 DYNAMIC ELECTRICITY AND MAGNETISM. 

sensitiveness as the telephone, is believed to be sufficient 
to account for the improved transmission; and observa- 
tion shows that continuous use produces a perceptible 
rise of temperature in a transmitter. This theory ap- 
plies more particularly to the microphone, but its appli- 
cation to carbon transmitters in general is obvious. 

The generally accepted theory of molar vibrations of 
the diaphragms, as already explained, is disputed by 
some, w^ho maintain that telephonic transmission is 
chiefly, if not wholly, due to molecular vibrations pro- 
duced by the variations of magnetism. In proof of this 
it is shown that such transmission is possible with in- 
struments constructed with thick disks, incapable of 
the molar vibrations ascribed to the thin ones. This 
theory also receives confirmation from the slight in- 
crease of length shown to be produced in a steel bar by 
magnetizing it, indicating that variations of magnetic 
strength in a magnetized bar must produce correspond- 
ing variations of length. The click due to magnetiza- 
tion, as observed by Page, also shows that magnetic, 
molecular vibrations may become sonorous. 

It is probable that all these theories are more or less 
applicable; that molar vibrations, molecular vibrations, 
and variations of resistance due to variations of tem- 
perature, all contribute to the observed results. The 
circular shape of the diaphragms and plates is also im- 
portant in contributing to evenness and regularity of 
vibration, and hence producing corresponding evenness 
and regularity in the atmospheric undulations, and 
should not be overlooked. 

Multiplex Telephony. — The ordinary conditions of 
telephonic transmission require that each subscriber 
should have a separate wire, since each must operate 
his own line, on which strict privacy is required, and 



THE TELEPHONE. 373 

be in constant connection with a central exchange 
through which he can be put in communication with 
others; while, in telegraphic transmission, the messages 
of numerous persons are sent, in rotation, from a central 
station, over the same wire, by experts, to whom alone 
their contents are known. Hence a telegraph line can 
be kept constantly occupied by thousands of persons, 
and rapid, automatic transmission employed, while a 
telephone line is occupied, usually, only a small propor- 
tion of the time by a single person. 

As this difference makes the expense of telephonic 
transmission enormous, as compared with telegraphic, 
various methods have been devised for simultaneous 
duplex telephonic transmission, similar to that of 
telegraphic, and also for the occupancy of the same 
line by several persons in rotation. While experiments 
of this kind have been, to some extent, successful, their 
success has not been such as to warrant their general, 
practical adoption. 

The occupancy of the same line by different sub- 
scribers in rotation is, however, in practical use for 
intercommunication between different central stations; 
the trunk lines, already referred to, being employed in 
this way; the proportionate number of such lines to the 
subscribers' lines being dependent on the amount of 
intercommunication. 

A description of the various experimental methods, 
referred to above, may be found in Preece and Maier's 
book on " The Telephone." 

Long Distance Telephony. — The multiplicity of iine.s 
required for the operation of a practical telephone sys- 
tem, as shown above, and the difficulty of overcoming 
resistance and induction so as to reproduce speech dis- 
tinctly at the terminals of long lines, has till recently 



374 DYNAMIC ELECTRICITY AND MAGNETISM. 

confined the use of the telephone chiefly to the limited 
areas of towns and cities. Experiments in long distance 
telephony were formerly made on telegraph lines; and 
as these were composed of single, grounded, iron wires, 
arranged in numerous parallel lines, in close proximity 
on the same poles, and hence subject to high mutual 
induction, the results obtained were not sufficiently 
encouraging to induce the investment of capital in in- 
dependent telephone lines, and the true causes of failure 
were not at first clearly perceived. 

The effects of induction beween parallel telephone 
lines in proximity, which causes a person with a receiver 
to his ear, waiting to be put in communication, to hear, 
indistinctly, conversation between others, is well known. 
But the fundamental difference between telephonic and 
telegraphic transmission, as already shown, aggravates 
this inductive effect, when lines used for both purposes 
are in proximity ; the strong, intermittent current on 
the telegraph line overcoming the light, undulatory 
current on the telephone line to such an extent as to 
interrupt the undulations and render transmitted speech 
indistinct, especially on long lines ; producing a con- 
tinuous, accompanying crackle. 

Van Rysselberghe's System.— This effect can be reduced 
by giving the telegraphic current an undulatory motion 
similar to that of the telephonic. This has been done 
by Van Rysselberghe, a Belgian electrician. He intro- 
duced two electromagnetic primary coiis, having iron 
cores, into the telegraph circuit, one between the battery 
and the key and the other between the key and the line, 
and passed the ground wire through a condenser. The 
rise and fall of the current at each intermission, caused 
by the charge and discharge of the condenser, in con- 
nection with the retardation due to self-induction and 



THE TELEPHONE. 375 

magnetic lag in the coils, produces an undulatory effect, 
similar to that in the ocean cable lines. By connecting 
the telephone apparatus with the line through a sepa- 
rate condenser, the same line can be used for simulta- 
neous telegraphic and telephonic transmission. 

As this system requires that all parallel lines, mounted 
on the same poles, shall be constructed in this manner, 
and shall also have special apparatus to prevent the tele- 
phonic induction referred to above, all of which entails 
considerable extra expense ; and as it also retards tele- 
graphic transmission, it has not come into general prac- 
tical use, though employed on some of the Belgian 
lines. 

The American System. — In the long distance telephone 
system now employed by the American Telephone and 
Telegraph Company the lines are composed of No. 12 
copper wire, and are complete metallic circuits, without 
ground connections ; each line having two wires, on 
each of which the current flows in opposite directions. 
The superior conductivity of copper, as compared with 
iron in one branch of the circuit, and the earth in the 
other, is apparent ; besides which the mutual induction 
of opposite currents in the two parallel lines proportion- 
ately increases the current strength in each direction ; 
while the freedom from grounded connections at the 
terminals prevents interruption from the inductive 
effects of contiguous, grounded, telegraph and tele- 
phone lines, usually numerous at such terminals. 

Where several such lines are mounted on the same 
poles, the mutual induction between currents, which 
would produce " cross talk " between the lines, is neu- 
tralized by introducing lines constructed with numer- 
ous transpositions between the straight lines. These 
tj^aaspositions are made at regular intervals of a few 



3/6 DYNAMIC ELECTRICITY AND MAGNETISM. 

poles apart, by crossing the two branches of the line, 
without contact, so that each takes the place of the 
other on the cross-arms. By this means the adjacent 
lines, on either side, are alternately brought into prox- 
imity, through short sectional distances, with wires bear- 
ing reversed currents in each section, and thus the effects 
of induction are neutralized. 

The instruments employed are the Running trans- 
mitter and the Bell receiver, with the signaling appa- 
ratus already described. 

The Hunning Transmitter. — The diaphragm of this 
transmitter is a disk of platinum foil, supported by a 
metal ring, and protected by wire guards in front. A 
thicker disk of brass, gold-plated, is placed back of this 
one, and parallel with it, at a distance of about -3^3 of an 
inch, and the space between filled with finely granulated 
carbon, sifted free from dust, whose superior quality as 
a transmitter has been already referred to. The whole 
is inclosed in a wooden box, to which is attached a 
metallic, funnel-shaped mouth-piece, in front, and, at 
the back, are attached the binding-posts for the battery 
terminals, one connected with the supporting ring of 
the diaphragm, and the other with the rear plate ; so 
that the current must pass through the carbon. 

Transmission on Long Distance Lines. — Lines 600 miles 
long are now in practical working order, speech being 
reproduced with distinctness, and lines 1000 miles in 
length are projected. On the line between Chicago and 
Milwaukee, 90 miles in length, whispered conversation 
and the ticking of a clock can be reproduced distinctly, 
also a hiss, the most difficult sound to transmit by the 
telephone. 

For the accommodation of the Bell Telephone Coni- 
pany's subscribers, connections are provided at the cen- 



THE TELEPHOXB, 2>77 

tral stations between their lines and the long distance 
lines, and the extra price for such service charged to the 
subscriber's account whenever such connection is made. 
But the reproduction of speech through such connec- 
tions, is not so perfect as by direct connection. 



INDEX, 



PACK 

Absolute Magnetic Intensity 43 

Accessory Apparatus, for the telephone 365 

Accumulator 235 

Action, heat developed by electrochemical 254 

' ' , local, in the battery cell 9 

Advantages of the Alternating Current Dynamo 189 

Agonic Line ... 38 

'' Map of the United States 52 

Agitation of the Solution, in electroplating 226 

Alliance Machine, the , 166 

Alternating Current Dynamo, advantages of the 189 

, the Gordon 185 

" ** " , " Westinghouse 186 

*• " Dynamos .,, 185 

'* ** " , separate excitation 189 

" '* Motor, the 198 

Aluminium, Bunsen's process for 229 

" , St. Clair Deville's process for 229 

" , the Hall process for 231 

Amalgamation of the Zinc, in battery cells 8 



American Morse Code 



312 



System of long distance telephony 375 

Ammeter, the Weston , ... 135 

Ammeters, gravity 145 



voltmeters and. 



134 

Ampere, the . . 6, 116 

Ampere-Hour, the 117 

379 



38o INDEX. 

PAGB 

Ampere's Table 82 

** Theory of Magnetism 89 

Rule 73 

Analogy between Magnetic and Electric Phenomena 67 

Analyzer, the, in the polarization of light 284 

Angles, measurement of , 122 

Angular ** " Deflective Force. 123 

Anions 207 

Annual and Diurnal Variation 50 

Anode, defined 207 

Anodes, the, in electroplating 221 

Apparatus, accessory, for the telephone 365 

" , signaling " " *' 366 

Arc, the 297 

" Lamp, the ... 297 

" Light, " 295 

" , multiple, defined 29 

Armature, of the magnet 55 

** , " " electromagnet 78 

" , " " dynamo 170 

" , " " " , the cylinder 174 

*' , " " " , Gramme, interior wire of the 173 

" , " " " , the Pacinnotti-Gramme 170 

** , " " " , " Siemens 167 

Armatures, of the dynamo, closed circuit and open circuit 176 

Armature's Magnetic Poles, location of the, in the dynamo 177 

Arrangement, station, in the telegraph 321 

Artificial Magnets 54 

Attraction and Repulsion, polar 58 

Astatic Galvanometer 1 29 

" Needle, the 73 

Automatic cut-out, for arc lamps 303 

" Rapid Transmission in telegraphy, the Wheatstone sys- 
tem 348 

" Regulation, in the arc lamp. ... 300 

Auxiliary Operations, in electroplating 223 

Ayrton and Perry's Spring Voltmeters and Ammeters. 142 

B. 

Balance, Coulomb's torsion .^ . 67 

ballistic Galvanometer. , 1 34 



INDEX. 381 

PAGE 

Battery, the, for the telegraph „ 314 

" , cell, element and 3 

*' , De La Rive's floating 84 

** Formation 28 

*' " , Two- fluid cells 23-34 

" , Grove's gas 233 

" .Sign 3 

'•^ , substitution of the dynamo for the, in telegraphing 347 

" , the voltaic. Definitions 1-12 

Becquerel's Discoveries in the electric relations of light 288 

Bell Telephone, the 361 

Bichromate Cell, potassium 15 

Bifilar Suspension 129 

Blot's Law o.. 44 

Bodies, paramagnetic and diamagnetic 64 

Blake Transmitter, the, for the telephone 364 

Blasting, electric 257 

Break, and make, in the electric circuit 93 

Bridge, the Wheatstone 157 

Brushes, dynamo 1 70 

*' " , position of the 178 

Buckling, conductivity and, in storage cells 246 

Bunsen Cell, the 26 

Button Repeater, the, in the telegraph 325 

C. 

Calibration of Galvanometer 124 

Callaud Cell, the 25 

Candles, electric 295 

Capacity of conductors, electro-thermal 255 

, storage, of storage cells 249 

Carbons, arc lamp 300 

Cardew Voltmeter, the 147 

Cations -»- 207 

Cathode " 207 

Cause of Deflection, of the needle 74 

Cautery, electric 258 

Cell, the Bunsen 26 

** , " Callaud 25 

", " Clark 139,142 

'* , '• Daniell 23 



382 INDEX, 

PAGE 

Cell, diamond-carbon 20 

" , Element, and Battery , 3 

' * , the Faure Storage 236 

** , " " " , defects of 240 

** , " ** " , improved 241 

** , •' " " , electric energy of 244 

" , " Grenet 16 

*' , "Grove 26 

* * , " Julien storage 247 

'* , " Law 20 

" , *' Leclanch^ 17 

" , " mercuric bisulphate 17 

*' , Plante's secondary 236 

", " " , electric energy of 239 

*' , potassium bichromate 15 

'* , the Pumpelly storage 247 

" , " silver chloride 27 

•' , Smee's 13 

" , theory of electric generation in the 6 

** , the voltaic, operation of 6 

" , Walker's 14 

Cells, connection between 33 

" , dry 21 

** , durability of storage 249 

" , gravity 25 

** , one-fluid 13-22 

" , polarization of one-fluid , 22 

** , two-fluid. Battery formation 23-34 

" , " " , construction of 23 

* , weight of storage 247 

" , zinc-carbon 13 

Charge and Discharge, effects of, on storage cell plates 245 

Charging and Discharging storage cells, relative time of 249 

Chemical Equivalence 216 

" Reaction in the Faure cell 240 

" "Plants"... 237 

Circuit, open, defined 19 

Clamping, insulation and, in the battery 9 

Closed Circuit and Open Circuit Armatures 176 

Code, the American Morse 312 

" , ** International Morse , ....?. 312 



INDEX, 383 

PAGE 

Coefficient of Magnetic Induction 76 

" "Magnetism q8 

" " Mutual Induction g5 

Coil, induction 98 

" a Converter, the 105 

* ' , spark 109 

Coils, resistance 155 

Common Galvanometers 134 

Commutation 165 

Commutator, dynamo 170 

" , " , improved 171 

" , RuhmkorfE's 104 

Compass, the mariner's , 35 

" , " surveyor's 37 

Compensating Magnet 74 

Composition of Grids, for storage cell plates 247 

Compound Winding, in the dynamo 183 

Compounds, electrolysis of mixed 212 

Condenser 99 

' * , Leyden Jar and 233 

" , " " as a 102 

* * , operation of i02 

Conditions of Electric Energy in the battery cell. 4 

" " Electrolysis 210 

Conductivity and Buckling, in storage cells 246 

" , insulation and , 112 

Conductor 113 

Conductors, electro-thermal capacity of . . . 255 

Connection Between Cells 33 

Connections, repeater, telegraph 327 

Consequent Poles in the magnet 61 

Constant Current Dynamo 183 

Potential " 183 

Construction of Core in induction Coil loi 

'* " " dynamo armature 174 

" " " " field-magnets 179 

** " " "electromagnets 76 

*' , line, in the telegraph 320 

*' and Operation of the Quadruplex 341 

" special, in induction coils 102 

" of Two-Fluid Cells 23 

Convection, effect of in electrolysis. .,,.,, ,.,.,...♦ 3ii3 



384 INDEX, 

PAGff 

Converter, the » 189 

" , the coil a 105 

** , '* Tesla Motor as a 202 

Core of dynamo armature 1 74 

'* field-magnets 179 

electromagnet 76 

induction coil 97, 98 

** '*, construction of loi 

'* ", induction of 97 

** " , sliding loi 

Cosine 122 

Cosmic Variation 51 

Coulomb, the 117 

Coulomb-Meter, the Forbes 150 

Coulomb's Torsion Balance 67 

Couronne de Tasses, the 2 

Crater and Point, the, in arc-light carbons 298 

Current, deflection by the electric 71 

" , direction of, in the dynamo 1 73 

*' , electric 5> 114 

" , " , deflection of by the magnet 82 

** and Electrylote, relative conditions of 219 

" , establishment of, in the arc lamp 299 

" , extra 96 

** , faradic, physiological effects of 107 

" , generation of, dependent on variation of intercepted mag- 
netic force 95 

** Induced by Another Current 91 

** " " Opening or Closing Primary Circuit 93 

" ** *• Magnet 90 

** " " Varying the Strength of Primary Current 94 

" Induction, results of 94 

" '* , rotary movement by 87 

" Meter, the Edison 150 

" , position and, in the incandescent lamp 305 

' * Reversal, effect of, in electrolysis. 217 

Currents, eddy 56, 196 

" , electric, generation of by induction 90 

** , Foucault 56, 196 

*• , mutual induction of electric 84 

Cut-out, automatic, for arc lamps 303 



WDEX. 3B5 

PAGE 

Cut-out, Ground-Switch, and Lightning Arrester, for telegraph. . . 319 

Cylinder, armature, in the dynamo 1 74 

D. 

Daniell Cell, the 23 

Declination 37 

Defects of the Faure Cell, 240 

Definitions. The voltaic battery I-12 

Deflective Force, angular measurement of 123 

Deflection, cause of 74 

" , by the Electric Current 71 

" , of " * * " by the Magnet 82 

" , magnetic force ascertained by 42 

De La Rive 's Floating Battery , 84 

Deposit, thickness of, in electroplating 225 

De Tasses, the Couronne 2 

Details of electroplating, various 220 

Development of the Electric Motor 190 

Diagrams, thermo-electric 266 

Dial Telegraph, the 357 

Diamagnetism, experiments in 79 

Diamagnetic Bodies, paramagnetic and 64 

' ' and paramagnetic substances, list of 81 

Diamond-Carbon Cell, the 20 

Differential Galvanometer 133 

Relay 333, 336 

Different Kinds of Electric Measurement. 118 

Dip, inclination or 40 

Diplex Transmission, in telegraphy 340 

Dipping Needle, the 40 

Direction of the Current, in the dynamo 173 

Discharge in Air and in Vacuo 107 

, charge and, effects on storage cell plates 245 

" , E. M . F. of, in storage cells 246 

Discharging Storage Cells, charging and, relative time of 249 

Discoveries, Becquerel's, in the electric relations of light 288 

, Faraday's " " " " " '* 284 

" of Gal van i " " battery i 

" , Kerr's, " ** electric relations of light 289 

" , Kiindt & Rontgen's, *' " " " 289 

♦* , Verdet's, in the ** ♦• '* " ,...287 



386 INDEX. 

PAGE 

Discoveries of VoJta, in the battery 2 

Disque Leclanche Cell 19 

Distribution, elevated road electric 204 

magnetic , lamellar 63 

, multiple series and series multiple 307 

of electric Power 203 

, parallel, in electric lighting 305 

, series, " '* *' 303 

, three wire system of, in electric lighting 308 

Double Reflection, effects of, on magneto-polarized light 292 

Dry Cells 21 

" Pile, Zamboni's 7 

Duplex Telegraphy 329 

" , the polar 333 

'* , '* " , operation of 338 

" , ** Steam's.. 330 

Durability of Storage Cells 249 

Dynamic Electricity Defined i 

Dynamo, the 168 

*' , advantages of the alternating current 189 

'* Brushes 170 

*' Commutator 170 

*' , constant current 183 

*' , " potential 183 

" , the Edison 185 

, " Gordon 185 

** and Motor, the 165-205 

** , the, as a Motor 193 

** , " , substitution of, for the battery in telegraphing 347 

** , *' Westinghouse 186 

Dynamos, alternating current 185 

E. 

Early History of the telegraph , 310 

•• " "■ " telephone 359 

Earth's Magnetic Poles, the. . 37 

Eddy Currents 56, 196 

Edison Current-Meter, the 150 

'* Dynamo, the 185 

*' Transmitter, the, for the telephone 363 

Effect of Convection in electrolysis 218 



INDEX. 387 

PAGE 

Effect of Current Reversal in electrolysis 217 

'* , the Peltier, in the electric relations of heat 268 

" , "Thomson, " " " " " 269 

Effects of Charge and Di -charge on storage cell plates 245 

** ** Double Reflection on magneto-polarized light 292 

"^ , Magnetic, in electrolysis 216 

Electric Blasting 257 

'* Candles 295 

** Cautery 258 

* Current 5, 114 

" " , deflection by the 71 

" *' , deflection of, by the magnet 82 

** Currents, generation of, by induction 90 

** ** , mutual induction of 84 

" Energy, conditions of, in the battery cell 4 

*' *• of improved Faure cell 244 

" ** , loss of, in the dynamo 195 

" " required for electroplating 225 

** Fuses 258 

" Gas-Lighting 109 

" Generation in the Cell, theory of 6 

" Heat to Electric Light, the relations of 279 

" Horse-Power, the. 118 

' * Intensity 29, 32 

Lighting 295 

" Measurement 1 10-164 

*' " , different kinds of 118 

*' Motor, development of the 190 

*' . Perforation ... , 107 

*' Potential no 

" Preparation of Plates for Faure Cell. . .' 244 

** " " Plants '* 236 

*' Pressure , in 

" Reduction of Ores. 228 

" Refining of Metals. 227 

" Resistance 5, 112 

*' " , measurement of 154 

'* Storage 233-251 

'* Telegraph, the 310-358 

'* Transmission, heat developed by 252 

'* Units 121 



388 INDEX. 



PAGE 

Electric Welding 273 

Electricity, dynamic defined i 

' ' , relations of, to heat 252-278 

"."light 279-309 

Electrochemical Action, heat developed by = 254 

** Equivalence 217 

Electrodes and Poles 4 

Electrodynamo meter, the Weber- Edelmann 152 

Electrolysis 206-232 

' * , conditions of 210 

* * of Mixed Compounds 212 

** J relations of, to heat 213 

** . secondary reaction in 211 

** of water 10,209 

Electrolyte, relative conditions of current and 219 

Electrolytes 206 

Electromagnet, the , 75 

Electromagnets, form of 78 

Electromagnetic Poles 75 

*' Saturation 78 

Electromagnetism 71-109 

Electrometers 119 

Eletcromotive Force 4, 1 1 1 

* * ** of discharge in storage cells 246 

** * * , lowest required in electrolysis. 214 

" " , Resistance, and Current, units of 6 

Electroplating 220 

•* , agitation of the solution 226 

** , the anodes 221 

** , auxiliary operations in 223 

" , plating solutions , .... 223 

" , required electric energy for , 225 

*• , " time of immersion and thickness of deposit. 225 

'* , various details 220 

ElectrorThermal Capacity of Conductors 255 

Electrotyping 226 

Element, and Battery, cell , 3 

Elevated Road Distribution 204 

Eleven Year Period, the 50 

Energy, electric, conditions of, in the battery cell 4 

, ** , of improved Faure cell 244 



<< 



INDEX. 389 

PACK 

Energy, electric, of Plants cell 239 

•• , *' , loss of in the dynamo 195 

" *•■ , required for electroplating., 225 

Equipment, simple line telegraph 314 

Equivalence, chemical 2i6 

" , electrochemical 217 

Equivalent, Joule's. 253 

Establishment of the Current, in the arc lamp. 299 

Exact Observ^ation, of the earth's magnetism. 51 

Exchange, the telephone 367 

Excitation, separate of the dynamo 189 

Experiments in Diamagnetism 79 

Extra Current 96 

F. 
Farad, the 118 

Faraday's Discoveries, in the electric relations of light 2S4 

' ' Laws, for electrolysis 215 

Faraday, nomenclature by > 206 

Faradic Current, physiological effects of 107 

Faults, location of, in submarine telegraph lines. 356 

Faure Cell, the , 239 

*' ", ", chemical reaction in 240 

** " , ** , defects of 240 

** ** , *' improved 241 

' * '* , * * ** I electric energy of 444 

Field, magnetic ..c 60 

** Magnets, the, in the dynamo 170,179 

Filament, the, in the incandescent lamp. 304 

' ' and Lamp Attachment 305 

Forbes' Coulomb-Meter, the 150 

Force, deflective, angular measurement of . 123 

electromotive 4, iii 

magnetic, ascertained by oscillation. , 42 

** , '* *' deflection 42 

*' , lines of , 59 

" , portative , 57 

'* , rabe of ,. c: ..,..,.:. . 60 

Formation, battery 28 

Form of Electromagnets , ., 78 

" "Magnets 63 



390 INDEX. 

>AGK 

Foucault Currents 56,196 

Fuses, electric 258 

G. 

Galvani, discoveries of i 

Galvanometer, astatic 129 

'* , ballistic 134 

** , calibration of 124 

** , differential 123 

" , sine 124 

'• .tangent 126 

" *• , the Helmholtz-Gaugain 128 

** , Thomson's reflecting 130 

Galvanometers 119 

" .common 134 

Galvanoscope, the 71 

Gas Battery, Grove's 233 

" Lighting, electric 109 

Gauss- Weber Portable Magnetometer, the 69 

Generation of Electric Currents by Induction . . . . 90 

*' '* Current Dependent on Variation of Intercepted 

Magnetic Force 95 

** , photo-electric 285 

*• , thermo-electric 259 

Generator, the magneto-electric 165 

Gonda Leclanche cell 19 

Gordon Dynamo, the 185 

Gramme Armature, interior wire of the 173 

Gravity Ammeters 145 

'* Cells 25 

Grenet Cell, the 16 

Grids for storage cell plates, composition of 247 

Grotthus' Theory of electrolysis 207 

Ground-Switch and Lightning Arrester, cut-out, in the. telegraph.. 319 

Grove Cell, the , 26 

Grove's Gas Battery 2<?3 

H. 

Hall Process for Aluminium, the 231 

Heat Developed by Electric Transmission 252 

« " ** Electrochemical Action 254 



INDEX. 391 

PAGE 

Heat, electric, relations of, to electric light 279 

'' and Light, in the arc lamp 299 

*' , relations of electricity to 252-278 

** , " " electrolysis to 213 

Hefner von Alteneck's Regulator. . . 302 

Helix of electromagnet 77 

Helmholtz-Gaugain Tangent Galvanometer, the 128 

History , early, of the telegraph 310 

" " ," ** telephone.,.. 359 

Horse-Power, the electric 118 

Hughes' Microphone 371 

Hunning Transmitter, the, for the telephone 376 

Hydrogen Alloy Theory, the, in electric storage 250 

I. 

Immersion, required time of, in electroplating 225 

Improved Commutator 171 

Faure Cell 241 

*' " *' , electric energy of 244 

" Transmitters, telephone 362 

Incandescent Lamp, the 303 

Inclination or Dip 40 

Induction, coefficient of magnetic 76 

*' , " ** mutual 95 

Coil 98 

*• of Core, in coil 97 

" " Electric Currents, mutual 84 

•* , generation of electric currents by 90 

** , magneto-crystallic ,,...... 64 

** , results of current 94 

** , rotary movement by current 87 

, self 96 

Insulation and Clamping, in batteries 9 

•* " Conductivity 112 

Insulator 113 

Intensity, absolute magnetic, the earth's 43 

Intensity, electric. . . , 29,32 

** , magnetic, the earth's 42 

Interior Wire of the Gramme Armature. . . 173 

International Morse Code, the , 312 

Interrupter, in the induction coil 99 

Inversion, thermo-electric. 269 



392 INDEX. 

PAGE 

Ions 207 

Isoclinic Lines 41 

Isodynamic Lines 44 

Isogenic Lines 41 

Isoclinic Map of the United States 49 

Isodynamic Map of the United States. 45 

Isogonic *• " " " *♦ 48 

J. 

Joule's Equivalent 253 

*' Law 253 

Julian Cell, the, storage 247 

K. 

Kerr's Discoveries, in the electric relations of light 289 

Kiindt and Rontgen's Discoveries, in the electric relations of 

light 289 

Key, the telegraph 314 

L. 

Ladd's Machine 170 

Lag, magnetic, in the dynamo 178 

Laminated Magnets 56 

Lamp, the arc 297 

" Attachment, filament and, in the incandescent lamp 305 

** , the incandescent 303 

Law, Biot's 44 

" Cell, the 20 

*' , Joule's 253 

" , Lenz's 93 

'* , Ohm's , 114 

Laws, Faraday's, for electrolysis 215 

Leclanche Cell, the 17 

Lenz's Law 93 

Leyden Jar as a Condenser 102 

** ** and ** , the ., 233 

Light, the arc. 295 

" , Becquerel's discoveries in the electric relations of 288 

*' , electric, the relations of electric heat to 279 

" , Faraday's discoveries in the electric relations of 284 

** , the heat and, in the arc lamp 299 

** , Kerr's discoveries in the electric relations of s 289 



INDEX. 393 

PAGE 

Light, Kiindtand Rontgen's discoveries in the electric relations of 289 

" , Verdet's discoveries in the electric relations of 287 

" , magneto-optic polarization 284 

" , Maxwell's theory of the electric relations of , 293 

" .molecular " " " ♦' *' *' 294 

" , photo-electric generation 280 

*' , polarization of 283 

" , the relations of electricity to 279-309 

" , strain in the media 294 

' ' , summary of the electric relations of 292 

Lighting, electric 295 

" " , the arc 297 

" , " "lamp 297 

'* ** , ** '* " , automatic cut- out 303 

•* , " *• " , " regulation 300 

" '* . *• *' •* , the carbons 300 

« , " *' *' , " crater and point 298 

** " , *' •* " , establishment of the current 299 

** *' , " *' ** , the heat and light 299 

" ** , ** *' " , Hefner von Alteneck's regulator. 302 

** " , ** " •* , series distribution 303 

" ** , *' *Might 295 

" *' , " incandescent lamp 303 

" *' J *' ** " , the filament 304 

" " , " ** •* , filament and lamp at- 
tachment 305 

" " , " ** •* , multiple series and se- 
ries multiple distribu- 
tion 307 

** " , ** ** ** , parallel distribution 305 

♦* " , *' '* " , three-wire system 308 

Lightning-Arrester, cut-out, ground-switch and, for the telegraph 319 

Line, agonic 38 

" construction, telegraph 320 

" Equipment, simple, in the telegraph 314 

Lines of Force, magnetic 59 

" " " , isoclinic 41 

" '* " , isodynamic 44 

" " " , isogonic 41 

" , long distance telephone, transmission on 376 

List of Diamagnetic and Paramagnetic Substances. ............ 81 



394 INDEX, 

PAGE 

Local Action, in the battery 9 

Location of the Armature's Magnetic Poles, in the dynamo 177 

** '* " Poles, in the magnet .1. 63 

Locating Faults, in submarine telegraph lines 356 

Lodestone, the 35 

Long Distance Telephone Lines, transmission on 376 

** ** Telephony 373 

Loss of Energy, in the dynamo 195 

** , magnetic, in magnets 56 

Lowest Required Electromotive Force, in electrolysis 214 

M. 

Machine, the Alliance 166 

" , Ladd's 170 

" , Wilde's 167 

Magnet, compensating 74 

*' , current induced by 90 

*' , the natural 35 

Magnets, artificial 54 

" , the field, in the dynamo 170, 179 

** , form of 63 

'* .laminated 56 

Magnetic Distribution, lamellar 63 

" Effects, in electrolysis 216 

•* and electric phenomena, analogy between 67 

Field 60 

* * Force Ascertained by Deflection 42 

'* " •• " Oscillation 42 

** ** , generation of current dependent on variation of 

intercepted 95 

•* Induction, coefficient of 76 

*• Intensity, the earth's 42 

** " , absolute, the earth's ^3 

** Lag, in the dynamo , 178 

•* Lines of Force c... 59 

'• Loss, in magnets , , , 56 

'* Maps c 40 

** *' , of the hemispheres , 39 

*' Penetration 63 

" Polarity 35 

'* Poles, the earth's,,.,..,.,...,.. ,......, 37 



INDEX. 395 

PAGE 

Magnetic Poles, location of the armature's, in the dynamo 177 

Saturation. , 55 

Shells 63 

" Storms 50 

" Strength, in the electromagnet 76 

Magnetism 35-70 

"■ , Ampere's theory of 89 

" ; coefl5cient of 98 

' * , origin of terrestrial 44 

" as a Mode of Molecular Motion 65 

" , residual 57 

*' , terrestrial, illustrated 41 

Magneto-Crystallic Induction 64 

Magneto-Electric Generator 165 

Magneto-Optic Polarization 284 

Magnetometer, the Gauss-Weber, portable 69 

Make and Break, in the electric circuit 93 

Map, agonic, of the United States 52 

" , isoclinic," " " " 49 

" , isodynamic, of the United States 45 

".isogenic, " " " " 48 

Maps, magnetic 40 

" , " , of the hemispheres 39 

Mariner's Compass, the \ 35 

Maxwell's Theory of the electric relations of light 293 

Measurement of Angles 122 

*' " Deflective Force, angular 123 

•* , electric 110-164 

** " , different kinds of 118 

** of " Resistance 154 

Media, strain in the, in the electric relations of light 294 

Megohm, the 116 

Mercuric Bisulphate Cell, the 17 

Metals, electric refining of 227 

Microfarad, the 118 

Microvolt, " 116 

Microphone, Hughes' 371 

Milliammeter, the Weston 138 

Milliampere, the 117 

Milliken Repeater, the, in the telegraph 326 

Mixed Compounds, electrolysis of 212 

Molecular Motion, magnetism as a mode of 65 



39^ INDEX, 

PAGE 

Molecular Theory of the electric relations of light 294 

Morse Code, the American 312 

" ** , " International , 312 

Motor, the alternating current „ 198 

'* , development of the electric 190 

** , the dynamo as a , 193 

** , /* " and 165-205 

" , principles of the 193 

*' as a converter, the Tesla 202 

" , the Westinghouse Tesla 199 

Motors, series, shunt, and compound wound 196 

•* , thermo-magnetic 204 

Multiple Arc defined 29 

'* Series and Series Multiple distribution 308 

" Switch- Board, the telephone 367 

Multiplier, the Schweigger 72 

Multiplex Telephony 372 

Mutual Induction of Electric Currents 84 

*' •' , coefficient of 95 

N. 

Natural Magnet, the 35 

Needle, the astatic 73 

*' , ** dipping.... 40 

** Telegraph, the 311 

Neutral Relay, the telegraph 342 

Nobili's Rings 216 

Nomenclature by Faraday 206 

O. 

Observation, exact, of the earth's magnetism 51 

Ohm, the 6, 1 1 6 

Ohm's Law 114 

One-Fluid Cells 13-22 

** ** " , polarization of , 22 

Open Circuit defined 19 

Operation of Condenser 102 

" the Polar Duplex 338 

** " " Quadruplex, construction and 341 

" "Voltaic Cell 6 

Operations, auxiliary, in electroplating. , 223 



INDEX. 397 

PAGE 

Ores, electric reduction of 228 

Origin of Terrestrial Magnetism c 44 

Oscillation, magnetic force ascertained by 42 

P. 

Pacinotti-Gramme Armature, the , 170 

Parallel Distribution, in electric lighting 305 

Paramagnetic and Diamagnetic Bodies 64 

Peltier Effect, the, in the electric relations of light 268 

Penetration, magnetic 63 

Perforation, electric 107 

Period, the eleven year 50 

Periods, secular 46 

Photo-Electric Generation 280 

*' " Reduction of Resistance in Selenium 282 

Photophone, the 282 

Physiological Effects of Faradic Current 107 

Pile, dry, Zamboni's 7 

" , the Voltaic 2 

Plant^'s Secondary Cell 236 

Plates for Faure cell, electric preparation of the 244 

** " Plante " , " " " " 236 

** ** storage cells, effects of charge and discharge on 245 

Plating Solutions 223 

Point, the crater and, in arc light carbons 298 

Polar Attraction and Repulsion 58 

** Duplex, the 333 

** *' , **, operation of 338 

Polarity, magnetic 35 

Polarization 9 

'* of One-Fluid Cells 22 

*' of Light 283 

*' , magneto-optic 284 

Polarized Relay, the 336 

Polarizer, the, in the polarization of light 284 

Pole-Changer, the 334 

Poles, consequent, in the magnet 61 

" , the earth's magnetic 37 

'* , electrodes and 4 

" , electromagnetic 75 

** , location of the, in magnets 63 



39^ INDEX, 

PAGE 

Position of the Brushes, in the dynamo 178 

** and current, of the incandescent lamp 305 

Portable Magnetometer, the Gauss-Weber 69 

Portative Force, magnetic 57 

Potassmm Bichromate Cell 15 

Potential, electric no 

Power, distribution of 203 

Pressure, electric in 

Primary Circuit, current induced by opening or closing 93 

" Current, " '* ** varying the strength of 94 

Principles of the Motor 193 

** <« '< Telephone ,. 360 

Printing Telegraphs 358 

Prism or Gonda Leclanche Cell 19 

Pumpelly Cell, the 247 

Q. 

Quadruplex, construction and operation of the 341 

** , repeating by the 347 

*' Telegraphy 340 

Quantity electric 29, 31, 32 

R. 

Rapid Transmission, the Wheatstone system of automatic, in 

telegraphy 348 

Reaction, chemical, in the Faure cell • 340 

" , " , " " Plante " 337 

" , secondary, in electrolysis. 211 

Recorder, siphon, Thomson's 353 

Reduction of Ores, electric 228 

Refining of Metals, " 227 

Reflection, effects of double, on magneto-polarized light 292 

Reflecting Galvanometer, Thomson's 130 

Register, the telegraph 315 

Regulation, automatic, in the arc lamp 300 

Regulator, Hefner von Aheneck's 302 

Relations of Electricity to Heat, the 252-278 

Relations of Electricity to Light, the 279-309 

" *' Electric Heat to Electric Light, the 279 

** " Electrolysis to Heat 213 



INDEX. 399 

PAGB 

Relay, the telegraph 317 

** ** " differential. 333.336 

" ** ** neutral .... 342 

*' , polarized » 336 

Rheostat, water = loi 

Relative Conditions of Current and Electrolyte, in electrolysis.. 219 

" Time of Charging and Discharging storage* cells 249 

Repeater Connections, telegraph. 327 

Repeater, the button 325 

" , " Milliken 326 

Repeaters, telegraph o 324 

Repeating by the Quadruplex 347 

Repulsion, pokr attraction and o. . 58 

Required Electric Energy, in electroplating 225 

" Time of Immersion and Thickness of Deposit in elec- 
troplating c o . 225 

Residual Magnetism 57 

Resistance Coils 155 

" , electric 5, 112 

** , " , measurement of 154 

" in Selenium, photo-electric reduction of , 282 

Results of Current Induction , 94 

Reversal, effect of current, in electrolysis 217 

" of Rotation, in the motor c 202 

Reversible " ** ** ** , 197 

Rings, Nobili's 216 

Rotary Movement by Current Induction 87 

Rotation, reversal of, in the motor 202 

'* , reversible , *' ** " 197 

Rules, Ampere's 73 

Ruhmkorff 's Commutator 104 

S. 

San Francisco, secular variation at 53 

Saturation, electromagnetic 78 

, magnetic 55 

Schewigger Multiplier, the 72 

Secondary Cell, Plante's 236 

** Reaction, in electrolysis 211 

Secular Periods ... 46 

" Variation ....,.,....,.., 46 



400 INDEX, 

PAGS 

Secular Variation at San Francisco 53 

** " in the United States .. 48 

** *' at Washington <, , , 51 

Selenium, photo-electric reduction of resistance in ,0.. 282 

Self-induction. . . ., 96 

Separate Excitation of the dynamo 189 

Series Distribution in electric lighting 303 

" multiple, multiple series and, distribution 307 

" , Shunt, and Compound Winding, in the dynamo 180 

*' , " , " «♦ Wound Motors 196 

Shells, magnetic 63 

Shunt and Compound Winding, Series 180 

Siemens Armature, the 167 

Sign, battery 3 

Signaling Apparatus, for the telephone , 366 

Silver Chloride Cell, the 27 

Simple Line Equipment, telegraph 314 

Sine defined 122 

•* Galvanometer 124 

Siphon Recorder, Thomson's 353 

Sliding Core, in the induction coil loi 

Smee's Cell 13 

Solenoid, the 83 

Solution, agitation of, in plating 226 

Solutions, plating , 223 

Sounder, the telegraph 316 

Spark, '* 97 

*' Coil 109 

Special Construction, in induction coils 102 

Station Arrangement, telegraph 321 

Stearns Duplex, the 330 

Storage Cell, composition of grids for plates 247 

" " , conductivity and buckling in 246 

** " , effects of charge and discharge on the plates 245 

" •*, preparation of the plates 244 

** *' , E. M. F. of discharge 246 

** ** , the Faure 239 

** ** , " *' , chemical reaction in 240 

** *' , ** " , defects of 240 

** •'* , hydrogen alloy theory , 250 

*' *•, improved Faure 241 



INDEX. \0\ 

PAGE 

Storage Cell, Improved Faure, electric energy of 244 

" " , " " , " preparation of the plates .. . 244 

*• " , the Julien 247 

'* " , Plante's, electric energy of 239 

** ", " , chemical reaction in 237 

** " , " , electric preparation of the plates 236 

" " , the Pumpelly 247 

" Cells, capacity of 249 

" " , durability of 249 

*' " 5 relative time of charging and discharging 249 

" ' * , weight of 247 

Storage, electric 233-25 1 

Storms, magnetic 50 

Strain in the Media, in the electric relations of light 294 

Strength, magnetic . 76 

Submarine Telegraphs 352 

Substitution of the Dynamo for the Battery, in the telegraph. . . . 347 

Summary of the relations of electricity to light 292 

Surveyor's Compass, the , 37 

Suspension, bifilar 129 

Switch-Board, the telegraph 324 

" " , " telephone multiple 367 

System of automatic rapid transmission, the Wheatstone 348 

" " long distance telephony, the American 375 

*' " " " " , Van Rysselberghe's 374 

'* , three-wire, in electric lighting 308 

T. 

Table, Ampere's 82 

" of Thermo- Electric Potential of Metals 264 

Tangent defined » 125 

" Galvanometer 126 

" " , the Helmholtz-Gaugain 128 

Telegraph, the electric 310-358 

" " " , the American Morse code 312 

** " " , " battery 314 

*' " '* , " button repeater 325 

^' ** " , cut-out, ground-switch, and lightning 

arrester 319 

" " " , construction and operation of the quad- 

ruplex 341 



402 



INDEX. 



Telegraph, the electric, the dial 357 

■* , early history of 310 

*' , the international Morse code 312 

'* , *' key , 314 

" , line construction 320 

" , locating faults in submarine lines 356 

•* , the Milliken repeater 326 

** , needle 311 

*' , operation of the polar duplex 338 

** , the polar duplex 333 

*• , " polarized relay 336 

" , " pole-changer 334 

" , printing telegraphs 358 

** . the register 315 

** , " relay , 317 

*• , repeater connections .. 327 

" , repeaters 323 

" , repeating by the quadruplex 347 

" , simple line equipment 314 

" , the sounder. , 316 

" , station arrangement 321 

" , the Stearns duplex 330 

" , substitution of the dynamo for the bat- 
tery 347 

" , switch-board 323 

'* , the Wheatstone system of automatic 

rapid transmission 348 

Telegraphs, printing 358 

** , submarine 352 

Telegraphy, diplex transmission in 340 

** , duplex 32g 

** , quadruplex 340 

Telephone, the 359-377 

, accessory apparatus 365 

the Bell , 361 

, early history of 359 

, the exchange 367 

, Hughes' Microphone 371 

, multiple switch-board 367 

, principles of 360 

, signaling apparatus 366 



INDEX. 403 

PAGE 

Telephone, the, theory of telephonic transmission 371 

*' ** , transmission on long distance lines 376 

" transmitter, the Blake 364 

" , " Edison 363 

*' '* > " Hunning 376 

" , the, improved transmitters 362 

Telephonic Transmission, theory of 371 

Telephony, long distance 373 

" , " '* , the American system 375 

" , ** " , Van Rysselberghe's system. . 374 

Multiplex 372 

Terrestrial Magnetism Illustrated 41 

" " , origin of 44 

Tesla Motor as a Converter, the 202 

" " , the Westinghouse 199 

Theory of Electric Generation in the Cell 6 

•* " Grotthus, in electrolysis ^ 207 

" , the hydrogen alloy, in electric storage 250 

" of Magnetism, Ampere's 89 

" , Maxwell's, of the electric relations of light 293 

" ., the molecular of the electric relations of light 294 

" of Telephonic Transmission 371 

Thermo-Electric Diagrams 266 

" " Generation 259 

•• " Inversion 269 

*• ** Potential of Metals, table of 264 

Thermo-Magnetic Motors 204 

Thermopile, the 270 

Thickness of Deposit, in electroplating. 225 

Thomson Effect, the, in the electric relations of heat 269 

Thomson's Reflecting Galvanometer 130 

Three-Wire System of distribution, in electric lighting 308 

Time of Immersion and Thickness of Deposit, in electroplating. 225 

Torsion Balance, Coulomb's 67 

Transformer, or converter, in the alternating current system. . . . 190 

Transmission, diplex, in telegraphy 340 

" , heat developed by electric 252 

'* on Long Distance Telephone Lines 376 

** , theory of telephonic 371 

" , the Wheatstone system of automatic rapid 348 

Transmitter, the Blake telephone 364 



404 INDEX, 

PAGB 

Transmitter, the Edison telephone 363 

, " Hunning " 376 

Transmitters, improved " 362 

Tube of Force 60 

Two-Fluid Cells. Battery Formation 23-34 

" " " , construction of 23 

U. 
Units, electric 121 

" of Electromotive Force, Resistance and Current 6 

V. 

Vacuo, discharge in air and in „.,,. 107 

Van Rysselberghe's System of long distance telephony 374 

Value of Volta's Discoveries 3 

Variations, annual and diurnal, in terrestrial magnetism 50 

" , cosmic ** ** ** 51 

** , secular '* " " 46 

" , " , at San Francisco 53 

" , " , in the United States 48 

" , " , at Washington 51 

Various Details of electroplating 220 

Verdet's Discoveries, in the electric relations of light 287 

Vibrator, in induction coil. 99 

Volt, the 6, 116 

Volt-ampere. 118 

Volta, discoveries of 2 

Volta's " , value of 3 

Voltaic Battery, the. Definitions 1-12 

" Cell, operation of the 6 

Pile, the 2 

Voltameter, the water 151 

Voltameters 151 

Voltmeter, theCardew. 147 

" , "Weston 135 

" , "Wirt 139 

Voltmeters and Ammeters. 134 

" '* '* , Ayrton & Perry's spring 142 

W. 

Walker's Cell 14 

Wdshington, secular variation at 51 



INDEX. 405 

PAGE 

Water, electrolysis of 10, 209 

" Rheostat — loi 

** Voltameter, the T51 

Watt, the 118 

Weber-Edelmann Electrodynamometer, the 152 

Wheatstone Bridge, the 157 

" System of Automatic Rapid Transmission, the 348 

Weight of Storage Cells 24 7 

Welding, electric 273 

Westinghouse Dynamo, the. 186 

" Tesla Motor, the 199 

Weston Ammeter, the 138 

" Milliammeter, the 138 

" Voltmeter, '* 135 

Wilde's Machine , 167 

Winding electromagnets 76 

'* , series, shunt, and compound 180 

Wirt Voltmeter, the 139 

Z. 

Zamboni's Dry Pile 7, 8 

Zinc, amalgamation of the, in battery cells... ., 8 

Zinc-Carbon Cells 13 



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^% A Creneral Catalogrne— 98 pagres— of Works in all branclics of 
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